Cubase 8.5.20 stalls during opening a session

When I open cubase and try to open certain sessions it will stall. But then a back up of the same file will open. Why would this be?

What is the last thing you see attempting to load in the project? Are there any third party plugins in the project? What operating system are you on?

Is there anything in common for the sessions that stall in comparison to those that don’t? Thank you.