Failing Proxmox node and unable to access /etc/pve means that you have broken pmxcf.

killall -9 pmxcf
systemctl restart pve-cluster

Analyse which services are stuck:

ps -eo pid,stat,comm,wchan:32 | grep ' D '

Restart services:

systemctl restart pvedaemon pveproxy pvescheduler

And in case it was stuck at updating certs (on which it got stuck in my case):

pvecm updatecerts --force

In case UI is still unavailable:

systemctl restart pvedaemon pveproxy pvescheduler
systemctl restart corosync

In my case, at this point it was fine.