Thanks Marco. That was actually the cause of the crash !
I do not know why it affected only these 3 notebooks and not all the others I have. But using your hint, I opened each notebook again in V9, changed the style sheet to something else, I picked
report style. Closed all notebooks. Opened them all now in V10, and there was no crash anymore.
Then in V10 changed the style sheet back from
default. Closed them. Now they all open fine in V10. The stylesheet setting is now fixed inside the notebook.
Why would V10 be looking for a style sheet in the V9 folder any way? Strange. But the problem is fixed now.
Thanks for your help and trying them.