Deja-Dup backup failed due to out of space condition on backup disk. It is indeed full, but I see many entries older than 1 month even though in the backup settings I indicate to keep backups for "at least a month." The setup dialog says, "Old backups will be kept for at least a month or until the backup location is low on space." It seems I should never see the out-of-space message in that case. Why isn't Deja_dup removing the older files (> 1 month) to make space for the new backup?
deja dup keeps always two full backups and their corresponding incremental backup chain. It only deletes old backups if there are more than two full backups. Maybe you don't have two full backups.