During a database backup or restore operation, the server cannot start a thread for the backup process.
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. A database restore is terminated.
Retry the command or wait for the server to retry the backup if it was started by the backup trigger. If the error persists, it may indicate a shortage of server memory.