Where is the back up of the corrupt preferences file saved?

I got the pop-up stating that my preferences file was corrupted and saved as Adobe Illustrator Prefs.err but I can't find it anywhere. I'm trying to establish if there is a way for it to be restored as all my starred fonts, custom document sizes and other preferences settings are now gone and this would be at least a good few hours of unnecessary work to rectify.

I'm using a Windows 10 machine for reference.