Hi John,
One reason for receiving ICM_HTTP_SSL_ERROR is your ERP Client Certificate is self-signed. As this, connection spans public internet so HCI don't accept any self-signed certificates request due to security reasons.
Hence, client certificate of ERP should not be a self- signed certificate instead should be a signed one by a valid/approved CA. It is recommended that the customer system uses one of these CAs.
Regards
sharad