User FBA cannot create forders in a library

In all the sites I am facing the same problem:

* I have windows users and FBA users who belong to the same group and share the same permissions (e.g. collaborate).

* When a windows user wants to create a folder in a documents library, he clicks on the ribbon "documents" tab and clicks again on the "Create folder" button. It works fine.

* When a FBA user (with the very same permissions) tries to click on the ribbon "documents" tab, both the "documents" and the "library" tabs hide and therefore he cannot create the folder. He can only upload files to already created folders.

I have been playing with the permissions and with different configuration options, but I can't find the solution.

As metioned before, the same problem happens in all the sites.

Any idea?

Thanks in advance