I just had a similar error, couldn't update. My DNS settings where ok.
The error was from this repo:
deb http://apt.syncthing.net/ syncthing release
It now use https, so I change it to https and everything is ok now so I can update. I read this with https in this thread I guess. (ryecoaaron posted it for the tvhead repo)
I can not understand how they change this repos without the possibility to update the repos automatically in source.list or omv-extra-org-custom.list. You have to edit those files for yourself.
An inexperienced user is left out without the possibility to do updates anymore.
I thought OMV has the goal to become an out of the box nas solution.