Well, I know there's an answer to a similar question here: Getting a 🚫 sometimes on boot, but this is slightly different.
My system is running on external drive. Actually on an external drive partition. After I installed Waves plugins I encountered this error for the first time. So I booted in my main system on internal drive and used repair on that partition by
sudo diskutil mountdisk force /dev/disk2 because partition didn't show up. That didn't help. So I used
diskutil repairvolume /dev/disk2s4 and after repairing it helped and I was able to boot normally. It happened twice and each time at firsrt getiing 🚫 and after repairing a volume I was working fine (I'm running on this system now).
I tried reinstalling macOS – did nothing. I have an old backup that's lacking some files. So I guess, if I backup now it's going to backup the whole drive including system folders errors. Any ideas how to fix this without erasing/restoring onanism?