Hey folks,

got a feedback from the GOG support:

"Hello,

My colleague is today not available.

After taking a closer look, we were able to reproduce the issue that might be simmilar to the one you are writing about. It is indeed OS-related issue, and indeed it's mentioned by our users in this topic:
gog.com/forum/general/old_games_not_launching_and_eating_100_of_the_cpu_on_windows_7/post1

Please try to follow these steps if you want to troubleshoot it:

Press WinKey+R, and in the Run window type:
regedit
Press Ok.
In registry editor navigate to:
HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\GameUX\ServiceLocation
Once in there change the value of server address in the Games chain to:
127.0.0.1

Please note that the gameUX server, assigned by Microsoft to games.metaservices.microsoft.com/games/SGamesWebService.asmx is no longer operational. This link is dead. Most likely Microsoft in the near future will provide an update for Windows 7 users to correct/clearance to no longer use that service/server entirely. Naturally if you do not wish to manually change this, you can wait for the official Windows 7 update that will address this issue.

Regards,
Dr Cat
GOG.com"

So it indeed was the Windows Game Explorer (gameux) that was causing these issues.

Well atleast now we have fix(es) for it.

NOTE: the workaround that was posted in sevenforums, that I linked in my precious post, also
works (atleast for me).