Ok, I think that actually is a bit complex to explain in this way.
First, I am attaching the database to make it quite clear.
We have two tabs: "Impostazioni" and "Volontariato”. The first one has only one field from which depends the second tab. The tab appears if "cb_attiva_volontariato" is set to "Si”.
In conditioned tab we have four "cascade" fields.
cb_campo_volontario_utente_area has only one value. Selecting it, cb_campo_volontario_pet_therapy_attivita_utente appears.
cb_campo_volontario_pet_therapy_attivita_utente has two values: "Conduttore del Cane" which will bring cb_campo_volontario_pet_therapy_tecnico_coadiutore and "Veterinario" which will bring cb_campo_volontario_pet_therapy_veterinario_qualif.
The problem is when you try to save the first time. This is not allowed and an error appears on the tab (such as when there are fields to be filled).
After several tests, I noticed that the problem seems to be in the condition of the tab, more precisely when you select "Si". In fact, if you try to put everything on a tab, it works.