I was attempting to add an SQL Server database and began receiving timeout errors when attempting to save. I debugged by putting the jar file on a server which I knew had access to the database in question. After several abortive attempts, none of which worked, I happened to try something out of desperation.
The Port Number shows a default of 1433.
I hand entered 1433 into the Port Number field.
Lo and behold, the system connected instantly. I can only assume that the internally used default port number for SQL server does not match what is displayed, although I cannot begin to guess why.