Got it working again! 
Some weird things happened along the way (couldn’t connect after reboot), but I can’t say for sure if it was my fault (held power button on NUC too long (?) → BIOS setup).
My cheat sheet for next time:
- Remove folder from storage settings.
- Stop server via UI (rock.local)
- Turn off NUC.
- Turn on NUC.
- Re-add folder to storage settings.
I just wanted to add a single MP3 and it took me 3 hours of admin-recovery time. 