After Registration, Page unavailable

Version: 9.0.2
OS: Ubuntu 20.04.4 LTS


i have an Problem during registration Process. When a user registrates himself by E-Mail and Password after pressing register, they get the following error page:

Page unavailable

Sorry, but the requested page is unavailable due to a server hiccup.

Our engineers have been notified, so check back later.

Did everybody have an solution for this error?