when i change the url, menu items get messed up

Let me exlpair a little more:

This is what i want to do: when the menu is generated, the module places it in the navigation menu, but i really want to move it to another menu, actually create one blank new menu just to place the taxonomy menu created.

If i leave the "Module page" to its default ("category"), everything works fine, i go to the menu management, clik "edit" for the menu item corresponding to the vocabulary and move it to to new empty menu, it actually moves that item with all its subitems. perfect.

But if i change the "Module page" to anything else, in my case: "productos", then i get a "page not found" error when i click "edit" in the menu management. that page not found error comes because the path for that "edit" link is changed to something like this:

admin/build/menu/item/edit/-381

when it should be:

admin/build/menu/item/edit/172