Yes! this should be, a token issue, my mean was referring to this when say "something about session" ... i'm often not so precise even if on topic, sorry.It looks like a tocken issue.
I will return over to check what may phpBB require to be passed to avoid this in some circumstances ...
Copied your html output, may something will come out, then if not i will take a look to phpBB login function and what/where it happen that can return this result ...