Your file is not fully modded as it should.
You should take fresh one from the latest phpBB and add the USU and zero dupe again (since that's the only two mods in use in there).

About your particular problem, I believe that the reason is :

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$viewtopic_url = append_sid("viewtopic.$phpEx", "f=$forum_id&t=$topic_id&start=$start" . ((strlen($u_sort_param)) ? "&$u_sort_param" : '') . (($highlight_match) ? "&hilit=$highlight" : ''));

Which should use $phpbb_root_path, like this (with the 3.0.8 update which should work from at least 3.0.7 too):

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$viewtopic_url = append_sid("{$phpbb_root_path}viewtopic.$phpEx", "f=$forum_id&t=$topic_id" . (($start == 0) ? '' : "&start=$start") . ((strlen($u_sort_param)) ? "&$u_sort_param" : '') . (($highlight_match) ? "&hilit=$highlight" : ''));

++