Known issue - conflict with jquery 1.4 jquery_update - due to JSON syntax

This is a KNOWN CORE ISSUE that is fixed in D7, but if using D6 with jquery_update past jquery 1.4, some complex JSON responses (such as the embedded forms that bigmenu returns) will fail. You may see the error, or it might just not work, depending on your browser and debug settings.
#1086098: D6 JSON is total garbage - Create RFC 4627 compliant HTML safe JSON #479368: D7: Create RFC compliant HTML safe JSON

If you must use jquery 1.4+ then a fix is only possible via a core patch (arg)
http://drupal.org/node/479368#comment-4686306
http://drupal.org/files/issues/drupal_json_encode_4.patch

It's a half-hearted backport form D7, but works. May need to be applied manually.