Some more details
I got the error in the settings menu where you can pick the existing .keystore file, and type a password. I had done this a few times before in the past when installing on new pc's. I will get you the exact error.
In Tools\Private Sign Key. Clicked Load, I hit browse, picked the file, open. Typed in the password, the hit ok.
I've attached an image of the error.
Sadly I don't think I have a backup. The file i found was it. I was in the middle of re-arranging things on a few 3TB hard drives so it's pretty hard and time consuming to back up such large amounts of stuff. Luckily it was mostly junk over the years though. Now I stick to storing all my stuff in the google drive folder just in case.