Has anyone encountered a endless saving screen after you’ve finished painting?
I’m going to leave this on for a while before doing anything, would hate to loose all the progress I’ve made… 
Yes, tracked it down to an imported vinyl group from FM5 that used small letters. When I originally imported the group. the small letter ‘a’ had disappeared (as it had on two other imports). I’d added them again in Forza 6, and saved the group fine. Basically, if you’ve got an imported vinyl group that was missing elements when you imported it, it might cause the issue.
Using these ‘glitched’ vinyl groups I had to redo on a BMW E36 M3 seemed to cause the save freeze you’ve had. Of course, I’d spent an hour doing the design first, and lost it (twice)! I only found out after redoing the car design and saving after each vinyl added - hence pinning it down to the glitched groups.
I have since removed my glitched imported vinyls from sharing in FM6 in case they cause issues for others.