Who to contact about ESTA refusal for previously overstaying partner of US citizen?

If you were told getting a green card would “forgive” your overstay, you were mis-informed. What you should have been told is that the overstay was irrelevant to a green card holder’s admissibility.

Admission is basically a determination of whether you will leave and not try to stay and/or work in the US without permission. When you had a green card, it was impossible for you to stay or work without permission, you had permission.

As an analogy, in a number of US States, the age of consent is 18, with an exception for married couples. Having sex in those states with someone 17 3/4 years old can land you in jail, if you’re not married to them. During the marriage, sex is legal in all states. If you get a divorce or annulment before that persons 18th birthday, the exception no longer applies. There’s no ex-spouse exception.

For admissibility, there’s no ex-green hard holder exception. While a green card hold you needed neither a visa nor a visa waiver, you are no longer a green card holder and no longer eligible for a waiver. You can try for a visa

I think they are correct that you are not eligible to use the Visa Waiver Program ever again. INA 217(a)(7) (8 USC 1187(a)(7)) says:

(a) Establishment of program

The Secretary of Homeland Security and the Secretary of State are
authorized to establish a program (hereinafter in this section
referred to as the “program”) under which the requirement of paragraph
(7)(B)(i)(II) of section 1182(a) of this title may be waived by the
Secretary of Homeland Security, in consultation with the Secretary of
State and in accordance with this section, in the case of an alien who
meets the following requirements:

(7) No previous violation

If the alien previously was admitted without a visa under this
section, the alien must not have failed to comply with the conditions
of any previous admission as such a nonimmigrant.

Since you previously violated the terms of the VWP, the US government is required by law to not let you use the VWP. The law doesn’t seem to give any government agency or officer the discretion to give you any exception and allow you to use the VWP even if they wanted to. Like the consulate said, you can apply for a B2 visitor visa to visit the US.

Yes, they should not have allowed you to enter on the VWP last December too, and it seems like it was a mistake. Your having gotten in on the VWP doesn’t prove anything about your eligibility for the VWP — it could have been a mistake, and a mistake doesn’t create new rights for you — your eligibility must ultimately be determined from the laws and regulations, and here it seems from the law that you are clearly ineligible.

I think you were misled by an incorrect notion of "forgiveness". Neither marriage to a US citizen nor applying for or getting a green card provides any "forgiveness" of any past immigration violations. What you heard is probably a poor description of the fact that being out of status and illegal employment do not affect eligibility for Adjustment of Status (the process of getting a green card from within the US) for someone in the Immediate Relative category (spouse, parent, or unmarried under-21 child of a US citizen). There is no "forgiveness" involved — any past violations and their consequences remain — it is just that certain violations do not factor into the eligibility requirements for the Adjustment of Status for some types of applicants. It’s like if someone with a DUI is eligible for food stamps — it doesn’t mean getting food stamps forgives the DUI — it just means that the DUI didn’t factor into the eligibility requirements of the food stamps; but if the DUI disqualifies you to drive a bus, then that restriction still remains.

You could contact a lawyer and attempt to challenge this. However that will not be quick or cheap and, as user102008 says, unlikely to succeed.

The best way is to apply for the visa. If you are stable and settled in Germany you will probably be granted a multi year visa, which is no more difficult to use than the VWP. This is the usual situation for people denied VWP use for technical reasons. Such visas can be expedited if you need to travel urgently.