Do you have some sort of automatic reboot? Or automatic Updates? Did you have it set to reboot and rebuild the database?
This has come up a few times, here is a very long post about it.
I will re-state my suggestion based on the most recent findings.
Use the diagnostic tool: Hubitat Diagnostic Tool | Hubitat Documentation
Click Download backups > download all your backups to your PC
Click Download latest version (may say you already have it)
Then Click Restore Previous Version
First you could try installing the newest version if not already on it.
If that does not work, roll back to an older version, 2.3.6 or older if you have one (if not there is an endpoint link you can use to download it)
If none of that works, do a soft reset, then restore from your newest downloaded backup. Does not seem to matter what firmware you are on at this point. (Restoring a backup does not change the firmware version, you can restore any backup to any firmware, does not need to match).
If still hanging up, try your next newest backup, etc... try all the backups newest to oldest to see if one will work.
Often times going back one or two backups is what fixes it.
