I had gotten this once before. At the sasme time I was never able to flush the cache either. I was finally given the "Founder" permissions for phpbb and everything was fixed. I hope that is all it is thats blocking you.

I had all that, it required having the comment about being logged in to fix ...

Your Portal Version: 1.0.0RC3
Your phpBB Type: other phpBB Distribution
MODs installed: No
Your knowledge: Basic Knowledge
Boardlink: http://www.tww18forum.net

PHP Version: 5.2.8
MySQL Version: 5.1.30

What have you done before the problem was there?
I installed the portal on my test ...