Zandronum

No matter what wad I'm running, or even if I'm running base Doom 2, I can not change any of the gameplay options, or "DMFlags". Remmeber, this is in single player.

A little error message pops up in the top right corner saying "DMFlags cannot be changed during a campaign."

I know I used to be able to change anything, no matter what I was playing. Is there anyway to fix this?

Strange, it works for me. Try starting Doomseeker and setup a server. Change the DMFlags to whatever you want then hit "Play Offline" at the bottom. Unless you wanted to change everything on the go, then I don't know.

There must be some wad in your skins folder, or otherwise being autoloaded somehow that contains a CMPGNINF lump inside. Incidentally, the "DMFlags cannot be changed during a campaign" must be the most annoying feature ever implemented.