Hmm. As you might know I'm on macOS...so this is pure guess...that said:
I can imagine two reasons:
- Some process wrote corrupted files
- A OS update changed something in your system's security settings
Things you might try concerning corrupted files (after each give MM another try):
- Delete all files in Documents/meshmixer/autosave
- Rename meshmixer.ini to something like "meshmixer_corr.ini" (you may want to keep it to compare it to a fresh one later on)
- If you added a part, stamp, stencil or shader material just before the issue occurred: Try to detect it in the libraries folders (Documents/meshmixer/libraries) and move it to some location outside Documents/meshmixer.
Concerning OS security settings you might try:
- Run Meshmixer as administrator (> right-click menu)
- Check your Documents directory if an application as Meshmixer has read/write permission to that folder.
- Move the whole meshmixer directory to a new location where you're sure that there's no read/write restrictions. As "meshmixer" isn't at the default path any more you should get a message on MM's start. Use the Redirect button to point to the new location of "meshmixer".
