It seems SE wishes to confound me yet again today.
So, this assembly worked fine without any errors as of thursday last week but now upon opening the assembly I get the following window. If I click ok all seems well until I try to save the file, when I hit save SE locks up and crashes without any crash reporting dialog box. The assembly shows no errors and everything is constrained and none show conflict (red). Checkin the remember box does not help either as I can't save the assembly for it to remember etc.