Just a few weeks ago I was playing around with C++ and created a function that changed the names of bones. Although everything worked, after autosaving a fatal error occurred and the project crashed. After that I couldn’t even open it.
To solve the problem I replaced the skeleton and its associated skeletal meshes with their uncorrupted versions from another third person template and that fixed the problem. I don’t think I did anything else, although I don’t remember now.