Because the steam forums seem to be ignoring my problem, I've come here for help, so I am well aware this is not the place to begin.
I recently forwarded my ports so I could host games on Garry's Mod for a few of my friends. I was successful in the fact that they could join.
However, only after forwarding my ports, I can not join any server on any game that runs on the Source Engine. I have my ports forwarded for Terraria and Killing Floor as well, but have not experienced any similar problems.
These are the ports that are open.
Does anybody have any idea what could be causing this?
Note that after I close those ports, I can join any server on any game.