it's caused because the user doesn't have a topics per page value set and tab_forums.inc.php assumes so..
ideally, it should check to see if it's defined.. if not, then grab the global config value.. if even that (for some reason) ain't happening, then there should be a catchall of 25 or whatever.
this occurs elsewhere too
you can 'fix it' without modifying code, by simply going into your preferences, selecting a value and saving..
