it's because you're not in an XGC usergroup. Specific clan forums are restricted to the people in that division. You can access these main forums because they are open to the public. You can't access any other clan forums until you join a usergroup.
If you were in XGC before try to answer these:
What was the name of the clan?
Who was the general?
Who was the division leader?
Did you leave, or did you disappaer due to your life taking priority?
Hope this helps!