I used Proxmox because it was free and open source with backup tools integrated into the system.

Interesting, when I used docker on a proxmox build, it would give me trouble. Once I installed it the normal way on an Ubuntu build, it was good to go.

I wonder why that is?

Fingers crossed that it continues to work for you in the current configuration!

Same here. I'm just installing it normally, and my nextcloud instance is just chugging along.