AlDee

I think I've confirmed a solution to this problem. I was having a similar issue with Version 4.2.1 on Windows 10. It seemed to install ok, and was stable, but then I tried editing some printer settings, and suddenly the program crashed and would not launch anymore. I tried uninstalling/reinstalling, but no use. I noticed a few binaries got left behind of the old versions and could not be deleted. Next stop, reboot. Now I was able to remove the files from Program files. Reinstalled, but no good. It wasn't until I read the posts here and someone mentioned the user profile that I looked at C:\Users\YOURUSERID\AppData\Local\cura\4.2\cache\definitions\4.2.1 directory I found that I could access the files but couldn't read any data. In comparrison, I had a previous 3.2 folder where I could read the files fine in notepad. So I renamed the cura folders under Appdata\Local, and Appdata\Roaming, and let the program restart. It worked fine, and let me start fresh. I don't know why it crashed, or if it will happen again but at least this is better than going through a lengthy uninstall/reinstall

Brilliant! I didn't realize that it needed to be in mm. Scaling the 3 axis in Cura by 25.4 also worked.