I'm bumping this thread because I still need help for this problem in PC Online mode. Clearly Olliebollie and I have the same problem, probably more people have it.
We can connect Online in every try.
We can enter any server all the time.
We can play quick matches all the time.
BUT WE NEVER EVER EVER EVER can play in a room, either hosting or entering, we get the "An error has occurred" message and that's it. I've just ran all the tests on PES09 Network Portal and everything is "A", everything says "You can play" when I really can't. In PES 2008 I had the exact same problem, which means that I haven't played online since PES6 (more than a year without playing PES online, that's right, imagine that). Please please reply to this if you have the same problem, what you did, any suggestion whatsoever!