Using a fresh install, I attempted to access my ACP repeatedly to no avail. I kept getting logged out of phpBB every time I tried to access the ./adm/ folder, and and received the error "Access to the Administration Control Panel is not allowed as you do not have administrative permissions." ... It seems my login credentials were being wiped for some reason? Perhaps that's deliberate, but one should at least be prompted to login again at that point.
I finally managed to get in, but only after clicking "Log me on automatically each visit" on the login screen.