What does invalid session mean?
It's just one of a series of error messages that this overworked forum gives you occasionally. Try back when the place is less busy. It's nothing to do with an error at your end, so there's not much you can do but live with it and hope that the forum will get better soon.
This happens often witch cheap ass servers and 56k users.
If you have a 56k, never write a post offline and reconnect to post. You will get invalid_session and your post will be lost.
Always do ctrl-c before posting to avoid such situation.
There IS a question in the FAQ (although it says page cannot be displayed) about this. It's because this forum is opperating under a much higher load then it was designed to.
It also can happen normally on this forum software, but only if you login, leave, come back like a day later, and then try to do something.