Have you tried doing the table alias without the "AS":

SELECT CLIENT.ID, USERADDRESS.FIRSTNAME AS USER_FIRSTNAME, USERADDRESS.LASTNAME AS USER_LASTNAME
FROM CLIENT
LEFT OUTER JOIN ADDRESS ON ADDRESS.ID = CLIENT.ADDRESS_ID
LEFT OUTER JOIN OPERATOR ON OPERATOR.ID = CLIENT.USER_ID
LEFT OUTER JOIN ADDRESS USERADDRESS ON USERADDRESS.ID = OPERATOR.ADDRESS_ID

Just a guess

Cheers