I've been getting new warnings this morning, when I attempt to log on.
After I enter my password, the warning pops up. If you refresh the page, or go to another website and return, the board will show you logged on. You can't just back-button back. When you log off, the warning will reappear, but you will be logged off when you return.
So, if you can't get to the board after you've logged on, then go somewhere else, and when you return you will be able to post.
Good grief. I finally managed to battle through the username and password hurdle. I also had begun to imagine I was causing offense to the system. It worked for me by reversing the order: filling the password in first and then the username. Mustn't have the knack yet.