Chances are there is still a space or line break before the forum script starts. You can't add text into phpbb directly from the php files, you have to edit the templates otherwise it will output before certain php functions are executed which modify the browsers headers (which has to be done before any output).

Either download a phpbb from their website and just reupload the index.php file (assuming you have no mods installed it shouldn't cause any problems), or if you are comfortable enough, open the index.php page in your file manager and make sure there is absolutely nothing (even white space) before <?php, and it should solve the problem.

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