Problem opening files

I am new to using iStopmotion and have created a handful of small files. Partway through working, I went to open a recent .mov file. It looked like it was opening, a small box flickered and then disappeared and the file didn’t open.
I have tried opening other files I created in iStopmotion and the same thing happens. If I go to Pathfinder and try to open from there, iStopmotion fires up and brings up the new file creation box.

I created the same films again by starting afresh with the same files, the file is saved but after closing it, it cannot be reopened. It was working fine up until recently. I am new to the software so could use some advise. Thanks.