2257775 - SUM error: The connection to the SL Controllers down

SUM runs into the following error during update:

The connection to the SL Controllers down.

The SDT GUI will reestablish the connection, when possible.

In the SDTServer.log, similar error message can be found:

Dec 10, 2015 11:38:29 AM [Info]: HTTP server started.
Dec 10, 2015 11:38:29 AM [Info]: SL Controller started.
Dec 10, 2015 2:12:08 PM [Error]:
com.sap.sdt.engine.core.communication.FactoryAlreadyRegisteredException: Factory for key 'com.sap.sdt.engine.core.communication.FactoryKey@178b846e' is already registered.

Dec 10, 2015 2:12:08 PM [Error]:
com.sap.sdt.engine.core.communication.FactoryAlreadyRegisteredException: Factory for key 'com.sap.sdt.engine.core.communication.FactoryKey@178b846e' is already registered.

Dec 10, 2015 2:12:08 PM [Error]: com.sap.sdt.server.core.ServerException==========

This symptom is often seen when you choose to continue the update after SUM reaches a user dialog screen and waits for user's input for a while. 

Normally, the symptom will be gone when you restart the SUM again. 

On all platform when SUM is executed. 

com.sap.sdt.engine.core.communication.FactoryAlreadyRegisteredException;SL Controller; SL server; SAPup; GUI; SL Common GUI;JSPM; offline deployment;java , KBA , BC-UPG-TLS-TLJ , Upgrade tools for Java , BC-UPG-TLS-TLA , Upgrade tools for ABAP , Problem