What do you think clicking the EULA is?
That's basically what I understand as "getting a license for a game engine" (this is also why Unity got popular in the first place, because they skipped all this nonsense).
OTH I accept probably 5..10 EULAs a week without thinking or even reading the text (most of them are not enforceable anyway).
There is nothing unique about German law here. The same is true in the USA and most other countries. If a EULA or any other contract (whether agreed verbally, signed physically, digitally, or via a "click") violates the law, it can be considered invalid/unenforceable. It has nothing to do with how the agreement was agreed to, but with what the agreement contains. (There may be some legal theory that may carry some weight that a contract agreed to via a "click" is more likely to be unconscionable that one agreed to with a physical signature, but that does not automatically make all "click" agreements unenforceable.)
Interestingly, Unity runs afoul of many of those.
You're still welcome to contact them directly and try to get some kind of contract directly if you can't agree to the EULA for access.