On trying to send a message  from BC 6.X to BC remote machine, in the audit logs we see the following error :
<<<
ERROR     RECEIVED_FROM_TP     Error sending request to Trading partner: Message: Internal Server Error
>>>

We verified that BusinessConnect engines and BusinessConnect Remote were running as Windows services on both machines and the ports were open for communication.

Windows environment

First, shut down the BC Remote service, and then start the BC remote service from command line, and re-test

The following messages in the Log Viewer indicate that messages are now flowing successfully:
<<<
PENDING      ACK_FROM _TP     Received acknowledgment from Trading Partner: Status: OK.
COMPLETED     SEND_TO_PP    Acknowledgement sent to private process.
>>>

With BC Remote now running successfully, uninstall and re-install the BC Remote Windows service, which picks up the current values in the BC Remote TRA file.  Do not just re-install the service, as a re-install will not overwrite the existing settings in the Windows Registry.
 

Re-install the BC remote windows service