Freshly registered to write my solution:
I have searched half the internet and changed the configuration files several times. Even on my parallel installed cloud I have not noticed any difference… with one tiny little difference: I had not yet entered an email address in the admin account. So Nextcloud couldn’t send an email (to where?) and spit out this cryptic error. I hope I could help some of you with this and maybe in the next versions the error message will be more precise or you will be forced to enter an email address directly during the installation.