Acutally there was no variable "RequireUserLogin" in the /settings/override/site.ini.append.php. I included it, set it to false, but got the same failure.
Is there a possibility to define a default user whenever the site is accessed. So I can set this user as anonymous and adapt his role.
But as I think about it, this wouldn't help. ezPublish would recognize him as a user and registering another person via the user site would be impossible.
Should I reinstall ezPublish...??
Any soon help is appreciated, because my work is lying down now.