Well, not officially.

Insulting the Führer or insulting Hitler, any laws for that?

'Insulting the Führer' was often interpreted as being a Lèse-majesté, codified from the beginning of the 2nd Reich in 1871 as "Majestätsbeleidigung und Verunglimpfung des Staatsoberhauptes (§§ 94 ff. RStGB, 90 StGB)", and still a German law now in §90 StGB.

After January 1933 the prosecution of such cases went up immediately.

It was especially enforced by the Treachery Act of 1934 (expanding a first attempt to outlaw 'criticism' from March 21, 1933, facsimile) which now called upon every citizen to not show any leniency and snitch on every occasion of such behaviour.

Especially paragraph 3 (in the 1933 version, 1934 it was updated to become paragraph 1) made it clear that:

Whoever deliberately makes or disseminates an untrue or grossly distorted assertion of a factual nature which is likely to cause serious damage to the welfare of the Reich or of a country or to the reputation of the government of the Reich or of a federal state or of the parties or associations behind these governments, shall be punished, unless other provisions threaten a severe penalty, with imprisonment for up to two years and, if he makes or disseminates the assertion publicly, with imprisonment for no less than three months. (2) If the act has caused serious damage to the Reich or to a country, a prison sentence may be imposed. (3) Anyone who commits the offence with gross negligence shall be punished with imprisonment for up to three months or with a fine.

The bold part may seem like doing just what the law wanted to punish: grossly mis-interpreting a factual assertion by shortening a quote out of context. And ironically it is of cause exactly just that what courts then went on to do.

This was also interpreted broadly, with a certain 'inclusiveness', where even refusing the nazi salute was punishable as insulting him. There are cases recorded for publicly insulting the Führer by voicing dissent, or voting the wrong way or leaving nazi organisations etc. There was some legal debate over classifying this perhaps more often as just "gross mischief" in order to not ennoble the misdeeds and their perpetrators, but also to avoid letting go of them completely unpunished.

Early on such insults could end 'quite well', like in 1935 Katharina Rothhaar being imprisoned for insulting for just the short time 5. Sep. 1935 – 7. Nov. 1935 In 1943 Karl Schmandt was sent to prison for a whole year. But often such a simple insult could also mean just being beaten to death in jail (like Leo Schneider in 1936). As time progressed and the regime seemingly solidified its power, these sentences showed a tendency to get harsher…

An interesting twist came when the legal debate started to see the avalanche of cases and considered separating insults directed at the office of Führer or at the person Hitler.

On 23. April 36 it was even declared by law that

"Insulting the Führer is not a punishable offence" [with indirect qualifier: as long as the to be expected punishment would not exceed 6 months]

("Act on the Grant of Impunity" ('Law Granting Exemption from Punishment')), (§2,2 in Reichsgesetzblatt 1936, RGBl 1936 I, S. 378.).

This meant that overnight in Prussia alone 414407 cases were dismissed in this amnesty. An opportunity for propaganda to 'really show how stable and tolerant the Nazis' were. (Hartmann, p226)

An official hint for interpretation stated that Hitler himself wasn't much interested in 'formal insults' but still wanted to get notified personally if someone claimed he would have been a deserter, corruption allegations and the like…

He would then give orders to go ahead, but that rarely happened. Thus the justice system used just all the other laws to protect the person, the office, the head of state, the party, the government, the members, the nazi uniform…

The number of laws used just increased ever more. No earlier one was ever repealed and the justice system tried very hard to circumvent the impunity act. They usually found their ways.

During the war such insults were then also prosecuted for being Wehrkraftzersetzung (~defeatism). Opportunities for punishment never decreased.

A certain Marianne Elise K., who worked as a technical draftswoman in Berlin, was not so lucky. A colleague in the armaments factory where she worked reported her to the authorities for telling the following joke:

Hitler and Göring are standing atop the Berlin radio tower. Hitler says he wants to do something to put a smile on Berliners’ faces. So Göring says: “Why don’t you jump?”

Marianne K. was hauled up in front of the notorious People’s Court, which had for some time been making a hated name for itself by handing out harsh sentences. The court’s president, Roland Freisler, who more than anyone else personified Nazi state judicial terror, sat in person over her trial. The judgment he rendered on June 26, 1943 read:

As the widow of a fallen German soldier, Marianne K. tried to undermine our will to manly defense and dedicated labor in the armaments sector toward victory by making malicious remarks about the Führer and the German people and by uttering the wish that we should lose the war. By these actions and the fact that she claims to be Czech when she is in fact German, she has excluded herself from the racial community. Her honor has been permanently destroyed and therefore she will be punished with death.

Marianne was executed by guillotine. The court rejected her defense that she was bitter about the fact that her husband had fallen in a senseless war of aggression. On the contrary, the court found that her status as a war widow aggravated her crimes.

— Rudolph Herzog: "Dead Funny: Humor in Hitler’s Germany", Melville House: New York: 2011.

The only example of a public figure making repeated insulting jokes (that I know of, corrections welcome) would be Werner Finck. Who once asked – pointing at giant pictures of Hitler and Goering on stage – that he didn't know what do do with those. Whether he should hang them or just put them up against the wall. While alerted Gestapo was visibly in the audience. Which he asked: "Do you get it? Or do you get me?" ("Kommen sie mit? Oder muss ich mitkommen?").

The "cultural monitors" from the Gestapo recorded in their protocol no. 41551/35II2C8057/35:

The Catacomb’s audience is largely made up of Jews, who frenetically applaud the common insults and biting, destructive criticism of the performer Werner Fink [sic!]. Fink is a typical cultural Bolshevik who apparently does not understand the new times, or in any case doesn’t want to, and who tries, in the same manner of earlier Jewish writers, to drag National Socialism and everything holy to National Socialists through the mud.
(Herzog)

He did in 1936 a 6 week stint in a concentration camp for that and antics like that, was released and prohibited from public performances. Which he did anyways. When a new arrest loomed in 1939 he 'fled to the Wehrmacht', frontline service. Goebbels tried to personally intervene for his release from the Wehrmacht into Gestapo hands again. But failed. (Based on Herzog's book "Heil Hitler, das Schwein ist tot - Humor unterm Hakenkreuz" ~"Heil Hitler, the pig is dead", which has a video online; re-enactment of Finck performing in his trial, after his release from KZ, starting at minute 12:30.)

The counter example would be Robert Dorsay. He was very fond of telling insulting jokes at parties. Like this one:

At a procession of the Führer through a city, young girls line the streets carrying flowers. One of them hands Hitler a bunch of grass. “What am I supposed to do with this?” Hitler asks. “Eat it,” the girl answers. “People are always saying that better times will only come when the Führer bites into grass.” (German idiom “to bite into grass,” meaning to push up daisies)

He was arrested, tried, sentenced to "two years re-education camp". Himmler intervened, thinking that way too lenient. Dorsay was re-tried, sentenced to death, executed in November 1943.
(Herzog)

Lettow-Vorbeck insulting Hitler?

The quote given in Wikipedia about Lettow-Vorbeck is the utmost pinnacle of dubiousness:

I doubt it's even correctly reported in the Miller book. All other instances seem to quote from him/Wikipedia…
Even 'if', Miller 'reports':

in the 60s — a (grand!) nephew — of another officer — said — something else, "f**k himself" being put in his mouth, a word the interviewee promptly doubted himself… well, what a whisper chain of hearsay. As a preliminary first, I'd like an inquiry of what exactly Paul von Lettow-Vorbeck did say? (Which might be unknowable either, but Wikipedia seems to rely on quicksand quoting of unreliable material). Others conflate this into "an official present" and 'Hitler's idea' (Thomas A. Crowson: "When Elephants Clash: A Critical Analysis of Major General Paul Emil Von Lettow-Vorbeck in the East African Theater of The Great War ", Master of Military Art and Science Thesis, 2003. PDF) or re-time it to 1937 (Robert Gaudi: "African Kaiser: General Paul von Lettow-Vorbeck and the Great War in Africa, 1914-1918", 2017. p415).

German sources seem to be much less colorful, rarely mention it, don't know that episode from anywhere else, cast doubt on the entire occasion, and simply report "if that offer happened, it is said that he declined…". In his own memoirs Lettow-Vorbeck never mentions any of this.

Furthermore: Meinertzhagen was the inspiration for James Bond, an all around fraud, claiming to have visited Berlin numerous times, once in 1939 meeting Hitler, with a loaded gun, to shoot him and Ribbentrop — when British visits were documented, but Meinertzhagen not mentioned, not confirmed anywhere else, and Hitler was indeed in Berchtesgaden at the alleged time. In 1935 there was no need for a new ambassador, etc.

Hitler's 'willingness to be offended'?

How much this may translate into insulting Hitler in private remains elusive. We just do not know much of that angle that's reliable.

But we do know that apart from attitudes towards legally defined insults, he was also of quite the short temper in general. He was so easily insulted as to have pressured his diplomats to persecute a Finnish dog (yes, the dog — and of course its owner) for insulting him:

This dog Jackie has its own Wikipedia page. (The affair is documented partially in the files of the archive of the Foreign Office, R 98974, 3/80 bis 4/94, 1941, Band 10.)

It is abundantly clear how arbitary and kafkasque this all was. A Herr Peter from a circus was asked by SA-men whether his chimpanzee could also salute properly. He couldn't. So his owner trained him to do so. The SA men were most delighted, told everyone about it. The monkey raised his arm whenever he saw any type of uniform. Then a sourface got wind of it. Petter was then put on trial and sent to the Russian front.

Another example: His temper primarily is even blamed for changing the entire occupation policy towards Denmark, after the Telegram Crisis. Because the Danish king did not write back enthusiastically enough after Hitler congratulated him on his birthday.

This is not to say that really everything that may have been insulting or everything that was brought to the attention of the police was leading to arrest and imprisonment (or even death, as envisioned in the law). Courts and their verdicts were often unpredictable. Or is arbitrary the better word?

A Jewish merchant (Kurt D., male!) introduced herself as the former "Jewish court lady of Adolf Hitler" to an 'Aryan' customer. Another 'Aryan' co-worker reported that. Her Jewish co-workers were questioned for not snitching on her and all went to jail for a short time. But the court found that the other officials overreacted, and that they simply didn't get the joke: The denouncer and the police didn't get the joke interpretation 'right' [evidently read by them as: "impertinent Jew insulting AH by alluding to h*m*sexual tendencies of the Führer…", said the state prosecutor (LAV NRW R, Gerichte Rep. 114 Nr. 6113)], and released them. For the time being.

(— Alexander Friedman: "Der ,Führer‘ und seine jüdische Hofdame. Flüsterwitze aus dem Rheinland der zweiten Hälfte der 1930er Jahre (am Beispiel des Archivbestands Sondergericht Düsseldorf", Medaon, Magazin für jüdisches Leben in Forschung und Bildung, 11, 2017. (PDF))

Jokes continued to be made, and those who told them risked a lot. But not always. In fact, in the early years the vast majority was let off with a warning or mild sentences. And the Gestapo only did catch a minority. Those they did get their hands on had to face arbitrary decisions.

— Andrea Hartmann: "Majestätsbeleidigung und Verunglimpfung des Staatsoberhauptes (§§ 94 ff. RStGB, 90 StGB): Reformdiskussion und Gesetzgebung seit dem 19. Jahrhundert", BWV, 2006. (pp224–238)

— Uwe Schulte-Varendorff: "Kolonialheld für Kaiser und Führer: General Lettow-Vorbeck — Mythos und Wirklichkeit", Ch. Links: Berlin, 2006.

— (Paul Emil) General von Lettow-Vorbeck: "Mein Leben", Koehlers: Biberach an der Riß, 1957.

— Wissenschaftlicher Dienst des Deutschen Bundestages: "Ausarbeitung: Überblick über den Stand der geschichtswissenschaftlichen Forschung zu Paul von Lettow-Vorbeck (1870–1964)", Ausarbeitung, WD 1 — 3000 — 061/14, Deutscher Bundestag: Berlin, 2016.

— Brian Garfield: "The Meinertzhagen Mystery. The Life and Legend of A Colossal Fraud", Potomac Books: Washington, 2007.