Well I had this problem and here is the solution that worked for me:
1. Go to www.leagueoflegends.com
2. If the top bar shows that you are logged in, click "Logout"
3. Clear your browser histories and cookies. Reference the instructions on the following website for your Browser of choice: http://kb.iu.edu/data/ahic.html
4. Close your Browser and re-open it.
5. Go to www.leagueoflegends.com and click on the Forum. Make sure you are still not logged in.
6. Go back to the www.leagueoflegends.com homepage and log back in (It is important that you do not attempt to login from the top bar while on the Forum page itself.)
7. Return to the Forum. You should now be able to access it normally.
8. If you continue having issues, please try logging into the forums from another Browser (if you normally use Firefox, try Chrome, if you normally use Internet Explorer, use Firefox, etc.)
Let me know if it worked for you, if another method worked or if you're still having problems and I will do my best to help you out!
