Sign Up Button in the Customer Portal doesnt work

[IT Service]

Hello Together we have a problem in our company with the ServiceDesk and namely customers can no longer register in the customer portal with their email.
When pressing the Sign Up button nothing happens and in the troubleshooting the error appears:
Failed to load resource: the server responded with a status of 400 ()
The Customer Permission Settings are set correctly and before that the whole thing worked without any problems. We are on the Jira Datecenter version 9.6.0.
The server has been restarted several times, unfortunately no improvement.
Any idea what could be the reason?

With kind regards