Looks that momentarily the server was having some problems. There's a lot of info about those "Cannot modify header information - headers already sent by " errors on the official phpBB site forum.
Why not check there to see if you get any idea of what's the problem? http://www.phpbb.com/support/
I've tried uploading an avatar not respecting the pixel size of 80 by 80 and the forum correctly displayed an error message formatted inside a cell, as it should be.