Unfortunatelly, I don't know what you are doing wrong and where to fix the error. We're investigating such kind of problems only for a fee.
The code shown is exactly the same on minibb test forums. For example, if you check this thread:
http://minibb.org/minibb-test.php?action=vthread&forum=2& topic=429you will see that
Moderator has a 'newbie' tag (as he has less than 300 posts), but
tester has no custom tag, but custom ranking "Test Account" is written in instead. As it is defined in bb_specials.php:
$userRanks=array(75=>'Test Account');