Twice in last month I've come across with this problem: I end a match, close the game, open it again to host but I can't though I know my ports are open. Thinking I forgot to go to join internet games first, which I know I have to before hosting for some reason for the game to work, I do and try again just to see that I can't get any player's ping either.
First time, remembering it happened to other people, I uninstalled the game without saving any info and installed it again and it fixed it.
Attached is my system.log if it's of any help.
Edit: Also, for some reason, the game task is still on even though I have already uninstalled it
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