I installed Omnipresence and tried to point to my Synology NAS via WebDAV as well; I ran into the same issue. I reported the problem to Synology using
their technical support inquiry form which might be more appropriate than the above-linked general feedback form.
I have not looked into darkermatter's solution yet, and am not certain I will - it clearly lies beyond the limits of my current technical abilities. I will probably end up using Omnigroup's server for now; I am trying to rid myself off of external cloud services, but oh well
