[Wed May 08 17:36:06.972634 2024] [so:warn] [pid 13101] AH01574: module php_module is already loaded, skipping
Led me to uninstalling the package php82-apache2 , which seemed to be still loaded. After that the other one (php83-apache2) seemed to be in use. (maybe some "reload" would have helped, too, i don't know..)
Then I had to install php83-session to be able to log in.
Now it looks okay in the admin page: