Empty taxonomy pages
I believe this question is too broad to have a correct answer. As noted in the comments, there are a few ways that this symptom could occur (i.e. modules that intervene), and tools to diagnose further (e.g. the sql query tool).

For my money, your best bet is to first figure out what code is actually handling the url - you might be able to do that with some debug code, or else by inspecting the menu tables (which are also routing tables in Drupal 6).