I just got some time to go and look at the showflat script, I guess the setting is in minutes (elsewhere we go by seconds, on other settings):
Code
		// Do they get an edit button?
		$edit_timer = $userob->check_access("forum","EDIT_POSTS",$topic_info['FORUM_ID']);
		$edit_now = time();
		$edit_window = $edit_timer * 60;
		$edit_good_until = $results[$i]['POST_POSTED_TIME'] + $edit_window;
		$still_editable = !$edit_timer ? true : ( $edit_now < $edit_good_until ? true : false) ;

Are you sure that every forums edit time field is set correctly? When you set the timer for 5 minutes on the admin group, does it display correctly for you?

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