Strange thing, my 9.5 project is now loading on 10.0.10, before it was crashing, and the only difference is I switched on Control Room and inserted a Sonarworks plugin on my monitor outs.
So maybe it doesn’t matter what plugin, but any plugin as an insert at the Control Room output? Very strange anyway 
) But the project is working now:)
EDIT: Just tested… Switched off that insert plugin, closed / opened that project again, it still works. Even switched off Control Room, close/open, still works… I’ll never know what the cure is, but I am happy, and sorry for this post.