»Backup failed« - what to do?

The DB is open and works fine. »Überprüfen & Reparieren« (don’t know how this is named in the english version) doesn’t show any damage but the »daily backup« says »backup failed«.
There is a lot of disk space available.

Anyone any suggestions?
Thank you.

Try Tools > Rebuild Database. When the procedure has finished, inspect the Log (Window > Log) for any file(s) that failed to be included in the rebuilt database.

Thank you, Bill.
It was a permission problem and Christian helped me to solve it.
(Though I really would like to know where, why and when the permission changed).
Now it runs perfectly again.