Broken after upgrade to 24.0.1.1

Hi!

Currently i´m using nextcloud 24.0.1.1 but after updating from 23.x I got an information about the unmet php-version (7.3). After upgrading to php 8.0 or 8.1 I get an error 500 or a white blank screen (depending if I use fast-cgi or php-fpm).

How is the best way to debug this situation?

Thank you for all your answers!

Regards, Martin