Today I had to move the Sonar database to another MS SQL instance and had to change the connectionstring. Since the new location was a named instance and is using a specific port number it took a while to get the sonar.jdbc.url correctly. Following Microsoft article got me on the right track.
Thought of sharing this information for others
Correct set up is:
Between the servername and instancename 2 backslashes are needed, one for escaping the special character.