Crashplan have just introduced a set of mandatory exclusions on both file extensions AND folder locations.
Anything deemed as operating system related (i.e. /srv - where the OMV3 data drive gets mounted) is no longer backed up.
Reddit /r/Crashplan has various posts of "Where are my backups?" from people, and I've now started the transition to Backblaze via Duplicati (install plugin, go...)
I will add that on OMV4, the /sharedfolders folder is not excluded - so you can switch backup src location to that folder and it will work - but your file extensions might still be subjected to exclusion (i.e. any .vmdk files are now excluded and so forth).
Hopefully this saves some of you a headache
(Mods : depending on your view, this might want a sticky or post in a "higher?" announcement forum - I'm not sure who may be using Crashplan here - but one other guy on Reddit is clearly also an OMV user)