Forum "Invalid Forum specified. If you followed a valid link, please notify the administrator"
 

I get this "Invalid Forum specified. If you followed a valid link, please notify the administrator" when visiting a forum-section with IE.
I use "simple user friendly-URLs" and that generates URLs with "a", "a" and "o" in the URL. This creates a problem! Because if I replace those chars with anything and press enter the forum-section loads up. How do I fix this? I need to be able to use "simple user friendly-URLs".