Roundcube is no longer working

On my Debian bookworm Roundcube (roundcube:all/bookworm-security 1.6.5+dfsg-1+deb12u9) Roundcube suddenly stopped to work.

All I get at https://mydomain.com/webmail is in the title "Internal error"
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Oops... something went wrong!
An internal error has occurred. Your request cannot be processed at this time.
For administrators: Please check the application and/or server error logs for more information.
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But sadly there are no entrys in /var/log/syslog, /var/log/roundcube/error.log or in journalctl -p debug --since "4 hours ago"|grep round

Does anyone have any idea where the service logs the errors?

Thanks for any hints!

 

could it be the /etc/roundcube/default.conf file has the wrong permissions or wrong owner?