Mac OS Bug Sur: suddenly stuck at 50% on profile loading screen!

My Macbook Pro (2017, Big Sur 11.0.1) won’t let me login into any of the 3 user profiles anymore. They all get stuck at 50% of the progress bar after entering the password. 

The last thing I did was working on moving around 20GB of pictures from the Photos library of one user profile to the other. I did that by exporting the pictures into a local shared folder, deleting them in the source library, then importing them in the target library and finally deleting the pictures from the shared folder. I recently had iCloud Photos enabled so the last thing I did was leaving the Macbook on overnight so the newly imported photos would upload to iCloud. 

When I checked in the morning, the Macbook was turned off (I guess it crashed). When turning it back on, the mentioned profile loading issue started.

What’s worth noting is that during the whole process, the internal 1TB SSD was constantly “on the edge” of being full. I only had 10GB free when I started the whole exercise, so I was quickly deleting the exported files (as well as the photos in the source library within the Photos app) and emptying the trash after I imported them in the new library. I constantly monitored the free space during the process but strangely, it hardly seemed to change at all. I only saw the file sizes of the Photos Library changing.

I once got the message in Photos that I cannot import the photos due to insufficient disk space, which I fixed by deleting some old iOS backup file. 

What I tried so far:

• reset NVRAM
• reset SMC
• Boot in safe mode
• Disabled Filevault (vis Recovery Mode -> Terminal -> resetpassword) and restarted several times
• Single User Mode (CMD-S), but that doesn’t work. I get a very brief black error screen which I managed to capture, see screenshot. Then I land on the regular login screen with my 3 local user profiles.

• In recovery mode I can mount the disk and see all my files. I deleted some big images via terminal to get additional free space (in case that was the issue), but that didn’t help
• Disk Utility First Aid gives me a snapshot is invalid message. See screenshot. 

Any other ideas?