The message "Connection unexpectantly closed by server" is coming from the Gen user exit TCPIPDynamicCoopFlowExit.java which by default makes 2 attempts to establish the connection to the server. Sometimes this message can indicate that more attempts need to be configured in the exit. However in this case the Transaction Enabler aefuf (funnel) log file showed it was reaching its 245 client connection limit i.e.
... ==>MAX USER CONNECTIONS 245 EXCEEDED, REJECTING CLIENT
... ==>Error:fdAlloc:No fd slots available!
... ==>aefuf: USER: UF fd=-1, rejected, MAX sockets
Therefore retry attempts would continue to be unsuccessful until restarting either the JVM instance or Transaction Enabler to clear all the client-server socket connections.