We have Sarah Wagenknecht to output that kind of drivel on Ukraine, no need to import Varoufakis. This ban is based on the same paragraph in the FreiG that the ban against that Austrian Nazi speaking at the AfD congress is based on: If you endanger the public order in Germany having a EU passport won't help you you still can't enter.

Though it has to be said that the Berlin (as in the state, not federal) police and interior ministry is very trigger-happy when it comes to Israel/Palestine, and I guess they are who issued the ban, press alas isn't being particularly detailed about any of this. Alongside with Varoufakis a couple of other planned speakers got the same treatment, among other things for saying that they would've participated in the October 7th attacks given the chance. Maybe he got caught in the cross-fire and courts will overturn the ban, maybe they won't, maybe he isn't even going to try and instead just milk the issue for political points.

EDIT: GOD FUCKING DAMNIT it seems that there never has been a ban in the first place and some police officer just made it up. The term you're looking for is "Dienstaufsichtsbeschwerde".