An error occurred whilst joining the channel

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A friend is trying to get online to join my game, but when he hits Online on the main screen he gets the above error. He is connected to PSN ok. Anyone know if it's just temporary / how to fix it?

EDIT: N.B. it's nothing to do with firewalls/port forwarding which is covered by the sticky.

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Followup: my friend never got into my game. I think it was just a server issue on Sony's end; I quit out of the game then fired it up again, and when I tried to go online I got the same error. I assume it was only down for an hour or so - I never went back to it last night.