This error might come for different scenarios. In my case this error occurred when i going to connect to the SQL server using BCS (Business connectivity services in SharePoint). though i provide correct user name and password the error is coming.
when i look in to the problem i found that SharePoint SQL database server is running on different port to protect unauthorized access.
But then how we going to connect ?
It is pretty simple but i wasted whole 3/4 hours to figure out that. normally we are giving port like Servername:port but unfortunately this is not working but if we give ‘comma’ rather than semicolon it is working fine.