From time to time I see this problem reoccurring, suppose you correctly setup an SQL Login for a certain account and when you tried to login it throws: Login failed for ‘RPAULO_WebUser’ (Microsoft SQL Server, Error: 18456) you might want to check whether the sql server supports mixed mode by going to SQL Server Properties of the server (connect via RDP).

Note that updating the settings would require a service restart to take effect.