Synology changed the format of .dss files between DSM 4.x and 5.x, who knows why the restore wasnt made backwardly compatible!!
There is a thread on the Synology forum (see http://forum.synology.com/enu/viewtopic.php?f=232&t=88917) that amongst various moans and stuff, contains some info that may help you 'construct' a DSM 5.x compatible .dss file from your configuration backup.
Of course, we all understand that a backup you cannot restore is not a backup at all - seems to have passed others by.......
Dave