(This sounds exactly like bug 50424, but that was marked fixed on 2013-07-15 with "deployed within the hour".)
I had made source edits to a mediawiki.org page over https. Looking at it 30+ minutes later I wanted to make one more quick edit, so I clicked [Edit]. VE worked fine, Save dialog, [Review your changes] worked, but the final [Save page] drew blue bars before doing nothing.
Firebug's network console showed the API requst visualeditoredit with
basetimestamp 20130723212405 format json minor 1 oldid 745108 page Manual:How_to_debug starttimestamp 20130723232059
was failing with
I had a similar occurrence a few days ago where the API reported badtoken. I can't remember the specifics of that, but I assumed it was my mistake because I was using multiple accounts on different wikis. This time I wasn't doing anything special. Connecting to https://www.mediawiki.org/wiki/Special:Version in another tab shows me as still logged in.
Maybe there are two bugs here: VE's inability to recover from this, and the lack of a "VE had an internal error" with suggestion as to how to what to do (e.g. copy the right-hand column of [Review changes]. Regarding the former, when I've lost state with source editing I usually find [Show preview] brings it back, maybe VE can implement something like this internally.