If you recently used the CP 3.5 or the ini patcher 3.5, there was a small typo that is causing the multiplayer master server fix to not work correctly. My recommended fix is to open your Nerf/System directory, delete your Nerf.ini, run the game once, then close the game and run this new version of the CP Ini Patcher found here.
The Community Pack 3.5 has also been updated to address this issue, though you will still need to delete your ini file and run the game if you used a previous version of the CP 3.5 or ini Patcher 3.5. New installs need not worry about this issue at all.