Sounds like a file has been locked, try stopping umbrel by running ./scripts/stop, then starting it backup ./scripts/start to see if the lock has been removed. If still same issue then there will be some process that has a lock on the file.
To find out from the command line run:
ps ax | grep macaroon
This will give you PIDs (i.e 67607) and paths to get an idea of what has a lock on that macaroon file.
You can then extract more info by running the following against the PID
lsof -p 67607
Hopefully this helps, let me know if you need more help