I think the only thing that will help now is a data backup.
Even if I cannot help you directly with your problem, I can share my experience with NC. There will also be some resentment, but I can’t tell if the problem is with NextCloud or UCS.
The constellation: NextCloud obtains its data from SMB shares (external storage) is for me the only sensible constellation in corporate use.
Updates are crap!
Sometimes the “external storage” is suddenly gone or the syntax of the connection data has to be changed. That was in the past. If NextCloud-Music with Subsonic for streaming one’s own music collection is added (as in my case) (I don’t know of any corporate applications), it becomes a catastrophe. Music is only played as sound rubbish and much more …
I run UCS+NC as a Proxmox VM. At the moment I have a combination (UCS+NC+Music) that works. I hardly dare to install an update on the system anymore. Every now and then I dare to do it and sometimes everything works afterwards. But before I do that I create a snapshot of the Next Cloud VM AND the UCS master (SMB shares). After the update, I test all the functions and if something doesn’t work, I roll back.
I want to express how fragile this construct is. I know that doesn’t help you with your current problem. But it might help you in the future. I don’t have such problems with the other UCS systems.