Incorrect quota operation for the user.
You can exceed the quota by moving files to the Recycle Bin and recovering from it.
Please create an issue on the bugtracker (https://github.com/nextcloud/server/issues) and don’t forget to put some more information about it:
- steps how to reproduce this error
- version of NC, configuration and logs.
It is not impossible that there are already similar reports, do a quick search there before posting it.