Been running OMV 2, did a clean install (not an update from 2->3) two different ways in an attempt to get past this.
Simplest case: installed OMV 3 fresh from ISO, no errors, tried to change session timeout on GUI. Get Error :Missing 'required' attribute 'enablessl'. See attached.
There are other similar errors when trying to change other values in the GUI (the attribute name changes), but let's ignore them since this is the simplest case and it's the very first thing I tried after clean install.
I've found similar errors on the forum but they were resolved with reinstalls. This is my 2nd install I think I'll stop here.