The reason of this error is that target database is not listening on the port that RCU is trying to use to connect to it. Please mention port number in the connection string.
1)If you have specified a port number in the connection string - check that the port number is correct – i.e. that it is the one that the database is listening on.
2)Specify the port number in the connection string by using the Port parameter.
To detect the port number of your Microsoft SQL Server instance, run regedit.
If you installed the default Microsoft SQL instance, the port number is located in the following registry subkey:
If you installed MSDE as a named instance, the port number is located in the following registry subkey:
HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\