Dropping this here in case anyone else runs into this issue and comes here looking for a solution.
I am hosting multiple Nextcloud servers on my Proxmox cluster. After updating a node to Proxmox V9 Nextcloud failed to run. This was the case with all of my servers. None of them would run on the V9 node.
Turns out there is a change to systemd in LXC on V9 that is breaking redis as it installed by default for me years ago. It tries to run as a private user but this is not allowed in an unprivileged container anymore.
The solution that worked for me was to run ‘systemctl edit redis-server’ and add this block to the override section at the top.
[Service]
PrivateUsers=no
Reloading the daemon and restarting the service restored functionality. I am not sure if there are other negative ramifications there and will be doing some further research, but if anyone gets hit by this I wanted to drop this here in case it helps.