This error means that your PDL does not recognize the string of bytes being sent it’s way.
Typically, messages are framed with “framing” characters that let cloverleaf know where a message begins and ends. This is typically the MLLP protocol.
Dial up your PDL EO to see the data that you’re getting. You’ll probably find that the vendor is not sending the MLLP framing chars, or is sending the wrong ones……
