strict warning: Creating default object from empty value... line 220.

This looks like it was fixed in D7 but may not have been backported to D6 (still present in current 6.x-dev).

Line 220 of forum.module throws a strict warning. Should be:

$node->taxonomy[arg(3)] = (object) array(
  'vid' => $vid,
  'tid' => arg(3),
);

Patch attached.