I've had this issue with a number of people trying to access my forum. They sign into the website & click to access the forum page. The forum then gives the topic "General", which they have to click on to access further sub categories. It is at this point that the forum instead asks for login details for the website. I have attached images which should give a better indication of what I mean.
I can access the sub categories, as I have administrator rights, but other people with other rights cannot.