Strange, but if I move one of the left join just before the join that causes the connection to be lost, all works fine, i.e.:

FROM WK_SELEZIONI
LEFT JOIN CONDOMINI ON (CONDOMINI.CONDOMINIO_ID=WK_SELEZIONI.CONDOMINIO_ID)
LEFT JOIN AMMINISTRATORI ON (CONDOMINI.AMMINISTRATORE_ID = AMMINISTRATORI.AMMINISTRATORE_ID)
INNER JOIN UNITA_IMMOBILIARI ON (UNITA_IMMOBILIARI.CONDOMINIO_ID = WK_SELEZIONI.CONDOMINIO_ID)
INNER JOIN UNITA_U_CLIENTI ON (UNITA_U_CLIENTI.UNITAIMM_ID = UNITA_IMMOBILIARI.UNITAIMM_ID)
INNER JOIN UICC_INFORMATI ON (UICC_INFORMATI.UICC_ID=UNITA_U_CLIENTI.UICC_ID)
LEFT JOIN CONDOMINI ON (CONDOMINI.CONDOMINIO_ID=WK_SELEZIONI.CONDOMINIO_ID) <---- moved here
INNER JOIN CLIENTI ON (UICC_INFORMATI.CLIENTE_ID=CLIENTI.CLIENTE_ID)

So this must be a bug that needs an urgent fix ;)
Regards
Marco Menardi