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<added>You need to remove session IDs from your URLs, if your version of phpbb has session IDs in them</added>
And of course, the slightest little error on a single page will cause havoc. Always save copies of the unedited files, and upload the revised ones one by one, so you will know which file the error was made in.
The best advice is to find that thread at phpbb.com about Google, and start reading :) That is precisely what I had to do.
phpbb is a hacker magnet
Most of the hacker/cracker problems are due to people not upgrading from versions 2.0.0 or earlier (there was a security flaw which allowed access to the admin panel) or from those who didn't remove the installation files from their server after completing their install or upgrade.