OK, so I am still fighting this... Here is what I just did:
I downloaded phpBB 3 RC7 again, and installed a fresh copy in a "test" folder. Its here: http://bcasparamedics.com/test_forum/
I made all the settings exactly the same as my live forum, and it installed perfectly. I then made all the database settings the same, so both boards are running off the same DB.
The test board runs fine, and NEVER throws this error.
So.... I backed up everything in my LIVE folder, then erased it all. I then moved the files (which are working fine) from the test folder to the live folder.
The board ran correctly, all the posts and users are there... BUT IT STILL GIVES THIS DAMN ERROR!
It just won't go away!
I have tried everything I can think of, and everything I can find on here... no luck... please help!