Who destroyed the leading brigade of the 6th Guards Tank corps?

This is all a bit murky, but to me it looks as if the most likely candidate is the infantry division Friedrich Ludwig Jahn, plus possibly indeed tank hunters on bicycles. The problems here are 1. that this kind of military history is very "special interest", i.e. you will find a lot of sources with somewhat low reliability and relatively few authoritative ones. 2. These days seem to have been very chaotic, and 3. these events are overshadowed by the battle of Halbe, which involved the very same area around a week later.

This Friedrich Ludwig Jahn infantry division was created at March 31st, 1945 and was centered in Jüterbog (around 40 km south of Berlin and about 40 km south-west of Zossen/Wünsdorf). Most of its soldiers were very young and green, and it was not considered ready for combat in mid-April.

At this point a map of the general area might be useful (the map source is openstreetmap.org):

I have marked a few places that will be mentioned below: Jüterbog (where the division was located until mid-April), Luckenwalde (on the road from Jüterbog to Berlin), Baruth (which the Red Army took on April 20th), Neuhof (without caption here, on the road from Baruth towards Zossen and then Berlin), Wünsdorf and Zossen (the OKW bunkers were located about halfway between Wünsdorf and Zossen), plus Kummersdorf, Töpchin and Teupitz. I have added also the captions for Wünsdorf and Neuhof.

Note that the area to the east and northeast of this map has lots of lakes and looks as it might be quite difficult for tanks. This more difficult area is where the German 9th army was still located at that time.

The situation on April 20th was that the 3rd guards tank army was trying to push northwards along what is now the B96 road (from Baruth towards Zossen), while the 4rd guards tank army further to the west was pushing towards Jüterbog and Luckenwalde along what is now the B101 road. In Soviet literature, this seems to be considered part of the Cotbus-Potsdam operation (named after two important towns that were captured in said operation).

So, what happened in Zossen?

This very local, though possibly not very authorative source mentions two areas of combat on April 21st, and probably the day before: Neuhof and Zossen itself.

Nach erbitterten Kämpfen um Neuhof am Wolziger See und Überwindung der sumpfigen Flemmingwiesen erreichen die Truppen des 6.sowjetischen Garde-Panzerkorps und des 9.mechanisierten Korps der 3.Garde-Panzerarmee die Linie Wünsdorf Zehrensdorf-Töpchin. Von hier aus gelingt schließlich der Durchbruch durch die entlang des Nottekanals aufgebaute deutsche Verteidigungslinie und in den Nachtstunden auch die Einnahme und Besetzung der Stadt Zossen.

After heavy fighting around Neuhof at the Wolziger lake and crossing the swampy Flemming meadows troops of the 6th guards tank corps and the 9th mechanized corps of the 3rd guard tanks army reach the line Wünsdorf-Zehrensdorf-Töpchin. From there they succeed in breaking through the German defensive lines established along the Notte canal and, during the night hours, the capture and occupation of the town of Zossen.

(The Flemmingwiesen are the open area around Baruth. Note how the capture of the bunkers itself is described as unproblematic)

The part about fighting between Baruth and Wünsdorf is corrobated by certain Russian websites which seem to display machine translations of collections of certain journal articles, but in a way that I do not feel confident to name any author or title:

In the area of ​​the city of Zossen, before the start of the Soviet offensive, there was a command post of the General Staff of the Nazi ground forces. A whole underground city was built here, which housed the departments and services of the headquarters. Therefore, the Nazis fenced Zossen with four lanes of powerful defensive structures. The depth of the Tsossensky defensive region reached 15 kilometers. The terrain, replete with wetlands, forests, lakes, favored the creation of a strong defense. On the roads, in inter-lake defiles, blockages were arranged, pillboxes were built, tanks were dug into the ground. The enemy turned the settlements into centers of resistance. The Tsossensky district was defended by troops with a total strength of up to an infantry division.

By 12 o'clock on April 20, the brigade of the 6th Tank Corps of the 3rd Guards Tank Army approached the city of Barut. An attempt by the forward detachments to seize the city on the move was not successful. Then the corps commander decided to allocate two brigades - the 53rd and 52nd, of which the first was to attack Barut from the southeast, and the second from the west, going behind enemy lines. After a short artillery raid, the tankers attacked the enemy. The Nazis could not withstand the onslaught, and at 13 o'clock Barut fell. With further advance north of Barut in the direction of Zossen, the tankers again met organized resistance. They had to successively break through the defensive lines of the enemy. The terrain limited the maneuver of tank units and made it difficult for them to advance. Only by the end of April 21, the entire defensive area was overcome, and on the night of April 22, Zossen was taken. When leaving Zossen, the Nazis blew up and flooded the underground structures that housed the headquarters.

Note that Tsossensky is just a declinated form of Tsossen = Zossen, and how the terrain the author describes (lakes etc) also fits the area around Neuhof quite well. It is actually possible even today to find tank ditches and traces of dug-in tanks in the area, but it is a bit hard to say whether they are from that time or from the era when Wünsdorf was the headquarters of the Western Group of the Soviet Armed Forces.

What are the references that make me suspect the Friedrich Ludwig Jahn division is responsible?

We have one mention in "Die Armee Wenck - Hitlers letzte Hoffnung" by Günther W. Gellermann on p.35:

Das Gren.Rgt. 1 erhielt am 16. April den Befehl zum Abmarsch nach Südwesten. Nach dem sowjetischen Durchbruch an der Oder wurde es auf LKW verladen und in den Raum Zossen verbracht. Hier hatte dieser Verband die Linie Kummersdorf–Teupitz gegen die aus dem Raum Cottbus vorstossenden Sowjets zu sperren. Bei diesem Einsatz unterstand das Regiment nicht mehr der Division, sondern der Kampfgruppe Oberst Oertel. Das Regiment wurde bei diesen Kämpfen vollständig zerschlagen.

The 1st grenadier regiment [of the Friedrich Ludwig Jahn division] received orders to march to the southwest on April 16th. After the Soviet breakthrough on the Oder river they were loaded onto lorries and brought into the Zossen area. Here this unit had to block the line Kummersdorf-Teupitz against the Soviets advancing from the Cotbus area. For this mission, the regiment was not subordinate to the division anymore, but to Kampfgruppe [colonel] Oertel. The regiment was completely annihilated in these engagements.

And another one (but without citing his sources) is Joachim Schulz, Die Schlacht um Berlin, p. 288:

Angesichts der Bedrohung der Stadt von Süden her wurde im Rahmen der sogenannten Armeegruppe Spree die noch in der Aufstellung befindliche Division Jahn - im wesentlichen zusammengesetzt aus jungen Arbeitsdienstmännern - auf etwa 40 km Frontbreite beiderseits Baruth mit Front nach Süden eingesetzt.

Doch schon am 20. April durchbrachen die Russen diese schwache Verteidigungslinie in der Nähe von Baruth. Feindliche Panzerspitzen gewannen die Gegend um Jüterbog und Wünsdorf.

Due to the threat to the city from the south, the Jahn division, which at the time was still in the process of being set up and mainly consisted of young Arbeitsdienst men, was deployed as part of Armeegruppe Spree on a 40 km wide south-facing front on both sides of Baruth.

But already on April 20th the Russians broke through this weak defensive line near Baruth. Enemy tank vanguards took the area around Jüterbog and Wünsdorf.

A third source that one might interpret in a way that places the Friedrich Ludwig Jahn division in that area at that time is given in Le Tissier, Slaughter at Halbe, without page number on google books:

On 29 April 1945 at about midday (the day of the abortive break-out attempt of several units west of us) three four-wheeld scout cars and about the same number of Volkswagen jeeps suddenly appeared before our positions [...] Their commander was a young second lieutenant, a Hitler Youth leader type. At first we took them for Seydlitz troops and wanted them to disarm immediately, but they identified themselves as a reconnaissance unit of the Friedrich Ludwig Jahn Division of the 12th Army coming from the area west of Teupitz. Their unit had become badly disorientated in the last few days and had contact neither to the west nor to the east. By the sounds of battle coming from the east, there must still be strong elements of our own troops around. Their commander wanted to know for sure and so had sent out his reconnaissance unit. The officer had radio contact, so we sent him on to the XI SS Panzer Corps battlegroup, but what happened then remains unknown. [...] This encounter took place in the woods three kilometers north-east of Halbe, north of the track to the Klein Hammer forest warden's lodge on the Dahme Flood Canal.

(Seydlitz troops were (rumoured?) German collaborators within the Red Army that pretended to be part of and dressed like the Wehrmacht)

So what about the bicycles? In an earlier comment, I had suspected this is a mistranslation of "RAD-Infanterie-Division Friedrich Ludwig Jahn", with "RAD" being translated to "bicycle" instead of "Reich Labour Service". It seems to turn out that at some point in those days in late April, we actually have bicycles (and clerks and drivers) and the Friedrich Ludwig Jahn division in the same place. But we are now quite far from what I would judge to be reliable sources. This forum entry (apparently based on "Die RAD-Infanterie-Division 'Friedrich Ludwig Jahn'" by Josef Pechmann) gives the elements of Armeegruppe Spree on April 21st as

Sie bestand aus den Resten der Division Jahn (1 1/2 Rgt.), dem Panzerverband Wünsdorf (6 Ausbildungspanzer), Truppen des Kampfkommandanten Zossen (etwa 1 Btl. Schreiber und Kraftfahrer, 3 Pz. Jagd — Brigaden auf Fahrrädern, hauptsächlich Hitlerjungen) und weiteren kleinen Alarmeinheiten.

It consisted of the rests of the Jahn division (one and a half regiments), the Wünsdorf tank unit (6 training tanks), troops of the Zossen command (about one bataillon clerks and drivers, 3 tank hunting brigades on bicycles, mostly Hitler youth), and further small ad-hoc units.

And another entry in another "special interest" forum purports to be the content of a document that lists a number of tank hunter units around Berlin, two or three of which might fit the area described here (only two match the date April 20st, all three match the date April 21st):

I. Im Einsatz befinden sich:

1.) Pz.Jgd.Verband 218 am 20.4., 2.4o Uhr eingesetzt im Raum Wünsdorf. Stärke 360 Köpfe.

2.) Pz.Jgd.Verband 2108 am 20.4., 2.4o Uhr eingesetzt im Raum Wünsdorf. Stärke 360 Köpfe.

3.) Pz.Jgd.Brgd. "Schneider" Gesamtstärke 1600 Köpfe

[...]

d) Pz.Jgd.Verband "U" am 21.4., 22.oo Uhr im Raum Rangsdorf eingesetzt.

"Translation":

I. Deployed are:

1.) Pz.Jgd.Verband 218 on April 20th, 2.4o AM deployed in the Wünsdorf area. Strength: 360 heads

2.) Pz.Jgd.Verband 2108 on April 20th, 2.4o AM deployed in the Wünsdorf area. Strength: 360 heads

3.) Pz.Jgd.Brgd. "Schneider" Total strength 1600 heads

[...]

d) Pz.Jgd.Verband "U" deployed on 21.4., 22.oo in the Rangsdorf area

(Rangsdorf is about 10 km north of Zossen)

If we believe that these documents are authentic, and that these documents are reasonably reliable, and that Pz.Jgd.Verband 218 and Pz.Jgd.Verband 2108 are 1. actually two different units and 2. actually are equipped with bicycles, it seems that these units would indeed be a significant part of the troops in that particular area.

Some further clues might be gleaned by looking into "19 Tage im Krieg" by Henry Schulze, which is unfortunately very rare, or by looking closely into the dates of death at the Soviet cemetery in Baruth.