From the upgrade instructions:
https://docs.eduvpn.org/server/v3/upgrade-debian-11-to-12.html
NOTE: there currently is a problem in which the apt full-upgrade removes vpn-user-portal. This is obviously wrong, but we have been unable to figure out the exact cause. Fortunately it is easy to fix this issue by installing the vpn-user-portal package again after the apt full-upgrade completes:
More debugging enabled when running full-upgrade:
https://paste.sr.ht/~fkooman/1822b75527a9ac0a906ca1944fab6ee306fae151
One can simply just install the vpn-user-portal package again and everything works:
https://paste.sr.ht/~fkooman/5663a9b09dcb2dc14dbe273451d57abb89738c42
Fresh installs on Debian 12 work perfectly fine.
We assume it has something to do with php-sqlite references in the
composer.jsonfile that is used in the Debian package to integrate the packaging with composer:"ext-pdo_sqlite"
Debian package description for vpn-user-portal: https://git.sr.ht/~fkooman/vpn-user-portal.deb/tree/v3/item/debian