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Trying to play with a good friend of mine. If we wait long enough at the home screen, "Firewall Problems Detected" shows up in the bottom right corner. If we try to join each others' game, this shows up while attempting to connect, then following up with "connection failed."

We added firewall exceptions, inbound and outbound rules for the game AND for the necessary ports, even tried playing with both our firewalls deactivated. No dice. Oddly, we played for the first time about a week and a half ago, had the same firewall issues text come up, but I was able to join his game anyway. Super frustrating. I've not come across any fixes other than the one above, and strangely, this issue doesn't seem to show up in any Torchlight II gog forum posts.
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I have the same problem. We're 3 players trying to log on the same game and one of us (A) can connect to the other two (B and C), but us two (B and C) can't connect to each other and to (A) when one of us (either B or C) is already logged in on a game hosted by (A).

Hope the explanation is clear. Oddly, (A) does not have the same Internet Provider as (B) and (C). (B) and (C) shares the same Internet provider. Could it be a router/modem problem?