I've found the issue:
I have 2 categories a main category and a nested category.
I don't want users be able to create groups in the main category, just in the sub-cats, so I've disabled the creation of groups in the main. But if I do that, the list of category disappear in the dropdown.
I must enable the creation of groups also in the main cat to be able to show the list in the creation form but I don't want that users can create groups in the main cat.
How can I solve that?