I'm on DO 12.22 x64, Windows version 2004, build 19041.685
When I'm in details view the icon next to a .ico file is that of the associated program (i.c. IconWorkShop), instead of the icon's thumbnail. IIRC it showed the thumb before a recent WIndows update. Also, in details + thumbnails view it does show the thumbnail.
How can I make DO show the thumbnail (again) in details view?
Do you see the same thing for .ico files in File Explorer when it is in Details mode?
If it only affects Opus, you may have the "Display generic icons..." option turned on in Opus.
If it affects File Explorer as well, something has probably messed up the .ico file type on your system. Comparing the registry data with another system where it's working is the direction I'd go in.
I've been a DO user for such a long time that I almost forgot about the existence of File Explorer (which is a good thing!) 
Checking FE I found that the thumbs aren't shown in ANY of the viewing modes. I guess you're right about something being messed up in the Registry. Unfortunately the Registry is such a horrific beast that it's not easy to find the right key, and cryptic names don't help either.
Do you (or any other viewer here) have any idea where in the Registry I should begin looking?
Thanx for the reply anyway. I'll keep you up to date.
In Opus, you can go to Settings > File Types, then use the File > Diagnostic menu at the top to create a dump of settings related to a type (.ico in this case).
You could then compare the results to another machine where things are working.
Beyond that, I'm not sure, since we're into the realms of Windows troubleshooting rather than Opus.