Yesterday i had the same problem on a SLES Linux Instanz.
Solution was to perform the update again using the OCC command commands.
After that, however, it shot up the graphics, which must be restored via design.
@Paul1:
you have to execute the command occ upgrade in your nextcloud folder on the server.
I also using a IONOS webspace and I had to execute the needed command as follow:
php8.0-cli occ upgrade
depending on your php version you have to adapt your version.