There should be a maintenance update (1.6.1) available soon from the theme repository that may take care of that. You can always try posting at the Graphene Support forum if it doesn’t.
Disabled all plugins , WordPress up to date.
Thanks for mentioning this, because it’s usually one of these two things. You’ve eliminated 90% of most problems right there.
Are you using a child theme, and if so, does your child theme have a functions file?
One last idea: try going into Graphene Options >> Display Options >> Miscellaneous Display Options and ticking the box labeled “Disable custom editor styles”. (This is a long shot, but it is simple, so it is worth a try.)