I keep getting an error trying to open an existing file

I’m trying to grade my students’ SketchUp assignments, and I was able to open the first one, but I can’t open anything else. I have the 30-day trial for educators, and I plan to pay for it afterward if I am assigned this class again. I have never used SketchUp before - it was sort of thrust upon me. I figured out the basics, enough to teach it. But now I can’t open any files. It’s only been maybe a week since I registered? I’ve signed out and signed back in, cleared my cookies, and looked through the help files. I can’t find anything to solve this. HELP!

Thank you.

Can you share one of those skp files so we can check to see that the file isn’t corrupted?