I've been using Painter for a few years, and haven't had trouble.
This year I upgraded from 2012 to 2019, and then again to 2020 when it came out.
When I upgraded from 12 to 19 nothing went wrong.
After upgrading from 2019 to 2020, all my .rif files in my explorer window, and when opening a file in the program are the same painter thumbnail icon!
I have a very chaotic filing system, and I relied heavily on the previews for sorting. (I'm going to fix it, promise!)
I've misplaced a file I need, and this is really frustrating that a program built for visual people won't show a preview of the image.
I'm running Win 10 home. I have both 2019 and 2020 installed
(because I'm slowly learning a new workflow in the new program.)
And, I found the fix elsewhere in this forum, and it is easy (Windows 10): Right-click on one of your RIF files, then click PROPERTIES. Click on the CHANGE button under OPENS WITH. Do not click on Keep Using; instead, scroll down until you find Painter 2020, then click on that. Click APPLY, and the thumbnails should re-appear.
I found this solution for Painter 2017, but it works for 2020 like a charm.
Hmm. Doesn't see to work on Win 7 unless I'm not understanding your instructions.