The game was working perfectly before, but now I am stifled with above error. I've tried every solution I could find and think of, ranging from reinstalling both steam and L4d, verifying the game cache within steam, updating graphics drivers, restarting, and setting all user permissions to full access.
I also tried launching the game from the folder itself, but then that posed another problem, as I would disconnect every 15 seconds with the "could not verify with steam blah blah blah" error" And the solution to this I found, was to launch the game within steam... -__-
As I said before, I did verify it. My problem is that right when I click on L4d2 in steam, I get the topic title error. It seems like it's an issue with steam if anything, since I can load up the game fine from my computer.