So I've been backing up our Macs to a Synology DS918 for a while now without any problems. Except for my old MacMini Server. It wasn't working, saying (roughly) that "the sparsebundle was in use, try to unmount it and try again". No obvious way to unmount it. Apple's fora are useless for things like this...

Turned out that the owner of the target sparsebundle was different from the user that was logged in to TM. Made them the same and everything is working fine now.

tl;dr - always check ownership and permissions first.

Cheers,
Scott.