After doing a desktop upgrade (From Debian 11 to Debian 12 (still in testing)), and upgrading my NAS, I also discovered my server was on Debian “oldstable” aka “buster”, with support ending in August 2022; ie, already past!
So after taking a snapshot I upgraded the server to Debian 11 aka “stable” aka “bullseye”.
The upgrade went quite smoothly, the only issue being that the mariadb-server package didn’t get installed for some reason, which left roundcube in an unconfigured state. Manually installing mariadb-server and resuming the upgrade fixed that.
A handful of configuration files needed some minor tweaking, then it was off to a reboot.
The system rebooted just fine, and a quick check of the most important services (email, cloud) seemed to show no problems.
Shortly afterwards I started getting some email notifications thoughl it seems that Nextcloud now requires php to have memory caching enabled. This required adding “apc.enable_cli” to be added to the php config files and all good.
Will see if it requires anything else over the next few days.
EDIT 2022-10-06 : exim4
Turns out the online-suggested fix broke my configuration using the vdom router.
All I needed to do is change $domain -> $domain_data to fix the “tained domain” errors, instead of taking the entire suggested fix.