Error message "Communication with the previous instance failed..."

We use eM client as mail client when we do send invoices to customers. Our invoice program does send the e-mails through eM client. We intiate the sending process in the invoice program. Almost some times every day eM client does show following error message:

“Commmunication with the previous instance failed. The other instance need to be closed to start the program”.

When this is shown the e-mails are not sent and they are not in the outbox in eM client. We have to initiate sending from invoice program again. And then it works without having done anything else.

Anyone have seen this problem?