I have played this game years ago, wanted to start a new game, and now GOG is telling me that it is an unknown version and the button to download / install it is greyed out. Anyone run into the same problem? It is the only game in my library which does that.

Thank you in advance!

Does the text above "unknown version" say "READY TO INSTALL" or something else?

The simplest thing would be to download the backup installer from the site or Galaxy and install it from there.

Verify / Repair your game.

It says ready to install under the grayed out buttons, yes, but I cannot pick repair/verify or anything other. Guess I'll have to download it manually via the backup files then, thank you.

Thought about that, but if the Play/Install button is greyed out (and probably says Install, not Play, from what the OP says), it would be mighty rare that the game was actually installed. No matter how messed up the files are, the Play button is always active when Galaxy detects the game as installed. I would also check the Galaxy installation while you are at it, if you use it for other games.

I installed the game on my Mac but now when I press play game it does nothing. I have reinstalled like 3 times. Thoughts?

Install it on a PC?

… Only slightly serious. ;P

You will have to add some details, like are you installing it through Galaxy, and do you have any mods installed? Is it the latest version? Your Mac should be powerful enough to run the game, but what hardware do you have? Can you repair the installation?