Check your logs on userman and the amount of disk space left.
To sort this out, make a backup of your userman data.
Remove Userman package.
Remove Userman store
Reinstall everything plus your backup.
On your new userman make sure all log's are disabled.
Same here we have around 50 mikrotiks with same issue we went back to 4.17, on X86 with corei7 also same problem I donnou whats going on with mikrotik, they lost us 2 days of work and customers complaint after upgrading to v5.20