Is it just me or in dosearch.php......
Shouldn't this:
$prevlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$prevpage&Limit=$Limit&Old=$Old">";
Be this:
$prevlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$prevpage&Limit=$Limit&Old=$Old&ByUsername=$ByUsername">";
... and you would need to do the same for the $nextlinkstart