If you have only 1 PLC (I assume defined in site S3?) connected to the network, then what is running at S4? It seems like you are accessing two sites simultaneously (S3 and S4) from computer 2? If so was the connection reset happen to S3 and S4 at about the same time?
Connection reset usually happens when the network is having some trouble continuing the connection, so one side of the connection decided to close the connection and the other side will receive a connection reset. In this case, XServer was the one that requested for the connection to be closed and hence the ExcelLink could no longer access XServer since the connection has been closed.
Were you able to recover the communication by stopping ExcelLink and clicking RUN button again? If so, then you may be able to restart the connection automatically by writing a macro in Excel to command the ExcelLink software to make connection.