I just Installed this and the menu doesn't open at all on the current latest version of the game, it's completely broken in the latest version of the game.

Solved: This was a Steam Proton on linux issue

To fix it, In steam, right click your game and go under it's properties. There should be a box for putting launch arguments. In that box put the following:

WINEDLLOVERRIDES="winhttp=n,b" %command%

Answer Taken from RainWorld Discord.