I just tried on a secondary account and there it works.
Back to my main account and it doesn’t work.
I already made a ticket but CCP told me that they can’t help me with this.
Deleting cookies, clearing cache and trying different browsers (Chrome/Opera/Firefox/Edge) didn’t work.
The only other related thing i found was an archived forum post from 2016. Someone had the same error messages and someone from CCP had to come in to fix it. :-/