Unfortunately, I can't send you the project file. With the new update I get a different error in the attachment.
I got around the issue by importing a JSON file I had of the project. Some images just didn't want to display despite them being present and correctly referenced for one skin, but I could correct it pretty quickly. My other 3.7 projects open just fine so it's only this one.
As for differences between files that work and don't work - the only thing that comes to mind is that I baked a lot of translation and rotation information of 8 different skins in animations themselves as keys on the first and last frame for the project that is not working. Beyond that the projects are almost identical.
