Received invalid response from server upon creating a new taxonomy term

We have a HS field on the Node Add form for a particular content type in Drupal 6.

It works fine just selecting an existing term.

However, when you try to add a new term and submit, it:

1. Adds the term
2. Returns 'Received an invalid response from the server.'
3. Clears all DOM elements from the page (so you end up with a white screen)

The 3rd AJAX call to hierarchical_select_json is the problem. The response is: 'Fatal error: Unsupported operand types in /var/www/htdocs/includes/common.inc on line 2999'

Watchdog gets 4 new entries when this happens:

1. Missing argument 2 for drupal_retrieve_form() in /var/www/htdocs/includes/form.inc on line 336.
2. call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /var/www/htdocs/includes/form.inc on line 382.
3. Argument 2 passed to xmlsitemap_node_form_alter() must be an array, null given in /var/www/htdocs/sites/all/modules/contrib/xmlsitemap/xmlsitemap_node/xmlsitemap_node.module on line 160.
4. uasort() expects parameter 1 to be array, boolean given in /var/www/htdocs/includes/common.inc on line 2997.

I've discovered that on this call that fails, _hierarchical_select_get_form_item($form, $name) returns nothing, which is then passed to drupal_render and causes the issue (since it's not a renderable array).

I've been troubleshooting this for a while and I'm at a loss. I think it must be some kind of interaction with another module, but I'm unsure which one. I'm hoping you may have some idea what is causing this, or how I can more effectively figure out what other module might be causing this behavior.