I try to post a reply, right after logging in and then going back to the page using back button of safari.

When I press the submit button of my post I get

You must select a forum in which to jump. Please go back and try again.

Please click [ Back ] to return to the previous page.

I don't understand this message and also not why the submit is not working. I'm already in a forum and I'm trying to post something in a specific thread.

If I click Back I get to the page where I just typed my reply to the topic. I can click Send again, only to get back to this error message.