Is this something that happens down the road? I've had nextcloud for probably 6 months now and while technically I've had a couple problems, they were self induced. I haven't heard of/seen maintenance mode.

Yes, I've been running it for about two years and it happened twice near the start due to automatic updates. In each case it was a couple of occ commands to clear it, but finding the command took quite a while.

I think it is happening less and less, it was more in the older days. I'm still on very old install (from somewhere barely after the OwnCloud split), so I saw some issues.

Of course, just before I was ready to wipe it all and start with the AIO container my partner started intensively using the calendar (switching from paper, finally!) So I need to spend some time with it and migrate properly without her having any downsides. When I do I will also set up EuroOffice and experiment with that. I'm rather looking forward to that. In that sense it's a real "lab" :)