Hi, I've been trying to install mods such as Chaos UT2 however, whenever I have installed them and try to start the game, I just get a popup saying my cd key is invalid.

So what do I need to do? Thanks in advance.

If you contact gog support they can give you a cd key, which you can also use for online multiplayer.

That said have you installed any version patches as well as mods? The game will be patched to the latest version. If you have just reinstall the game and install only the mods.

I got the same popup after patching the game thinking it might solve another issue I had. Silly me. GOG installs the latest version, 3369. Uninstalling and reinstalling the game resolved the issue.

Now, I don't know if this is necessary, but ChaosUT2 can be installed without running the ut4mod file. I used the UT2mod Extractor 2.0 (use at your own risk) to extract all the files for Chaos. (Extract with directory structure intact, then paste the files to your UT2004 directory) Worked like a charm. Be advised that for the ut2mod extractor to even see the Chaos file, you need to rename the extension from ut4mod to ut2mod. FTR, I was using the last version of ChaosUT2, v3.76