This message would indicate an error with the SQL connection. It's a rather large amount of data you are trying to load so I would check the package works with less records and then increase the number of records until you get the failure. How long does it take to load? Maybe there is a timeout error somewhere in between the application server and the SQL Server.
Also check the SQL Server log for errors at the Windows Application and System logs on the application server as it may give you additional clues.
I have seen similar errors occurring while running DM packages when a SQL server backup was performed at the same time.
See also
https://service.sap.com/sap/support/notes/2228401
https://service.sap.com/sap/support/notes/1664093
Stefan