Redirect problem - phpBB3 Drupal 5.2

I feel like you are getting a bit too trigger happy. If you are sure that changing the .htaccess file would fix this, you must also have a clear idea of what needs to be changed?

RewriteCond %{QUERY_STRING} ^mode=login(.)$
RewriteCond %{QUERY_STRING} !^bypass=true(.)$
RewriteRule ^ucp.php(.*)$ ../phpbb/login [L]

I wouldnt be surprised if it doesnt deal with the "..", it does however surprise me if the "phpbb" part of the rewrite is ignored.

Note that this only affects the rewrite of the login form, the redirect after a login is handled by the module, not the .htaccess file.