I am having trouble opening my game as well. It worked fine last night when I saved. I tried to open today and I get an error. This was saved to cloud, so I was able to download a previous version that I hope will help restore my work.
However the download of the c3p file gives me a zip file. Unzipping gives me everything spilled out into directories and a c3proj file.
Trying to open this c3proj file doesn't work.
Coming here to the forum and searching for the term brings me to this topic.
And now I'm stuck again. I posted already in the bug section about the error. I'm trying now to solve around it since no response and this topic also isn't that helpful. I just want to open my file. It's there and saved.