The Repository URL http://download.owncloud.org/download/repositories/stable/ apparently switched back to the 9.1.5 release - I installed 10.0 a few days back and now, when I checked if there is an "Ubuntu 17.04" folder I noticed, that there is now the previous version...
Just use http://download.owncloud.org/download/repositories/10.0/ for now which contains the version you're expecting.
I know that there is a workaround
But it seems that the link is auto-generated regulary, so I just wanted to bring this to attention...
The "stable" repository just contained the 10.0.0 accidentally and its expected that it now contains 9.1.5 again as 10.0.0 doesn't match this:
This channel delivers the latest feature releases ready for home users at minimal risk.
They are mostly waiting a few minor releases (e.g. 10.0.1 or 10.0.2) of a new major release until it is pushed to the "stable" repository.