My original Observer version is still in my library to download after buying Redux. So either they fixed it, or just make sure to search for ">observer" and not just "observer" in your library.
Nice ... who would have guessed. I tried "_observer" (which is obviously wrong) and observer, but not ">observer"
Just played around with the search engine for a bit.
Looks like it only checks exact matches at the beginning of words if you don't use wildcards.
So "server" won't find any matches, even if the sequence is present in at least 2 games, "*server" however does. It's a bit embarassing that after all these years and way over 1000 games on GOG I didn't learn that on my own :D
Thank you for your input.