Does it happen immediately when you click the RUN button, or does it run properly for sometime and then it happens?
When you preform online monitoring using TRiLOGI software, do you encounter similar error? You could have both ExcelLink and TRiLOGI monitoring the PLC at the same time. Does TRiLOGI report communication error occasionally too? If so, It is probably because the ExcelLink program receive a bad command somewhere which caused an exception that may not have been caught properly and therefore the program could not recover automatically.
Some USB to RS232 converter may appear to work when programming with TRiLOGI but may occasionally mishandle the communication and present a problem to ExcelLink. TRiLOGI is more forgiving towards occasional comm string error.
A beta version of TRi-ExcelLink is being developed to be more tolerant towards exception handling. you can contact support@tri-plc.com if you wish to try it. Please provide all purchase information (date or purchase, invoice number, company name etc that identify your purchase). We should be able to send you the beta version for testing if this is the cause of the problem.