During a database backup or restore operation, the server runs out of database space.
If the database backup was started with the BACKUP DB command, the database backup is ended and server operation continues. If the database backup was triggered automatically, the backup stops and the server waits for the retry period to expire and then tries the database backup again. The database restore process attempts to set the database capacity to the required capacity to hold the database being restored. If this fails, the restore operation is ended.
For database backup, make more server database space available, if necessary. For database restore, make sure that the size of the online database is at least the same capacity as the database being restored.