Error while sending the message "InvoiceDataToCheck"

What causes this issue?

When it checks the header data for the agency document, the system sends
the XI message #InvoiceDataToCheck#. An error occurred while sending
this message.
Error code: &SYST-MSGV1&
Text: &SYST-MSGV2&
Context: &SYST-MSGV3&

System Response

The system did not send the message and did not check the payment refere
nce in an external system.

How to fix this error?

If you do not want the system to perform an external check, you must dea
ctivate the <ZK>Check Payment Reference Externally</> indicator in
Customizing for Full Service Leasing (Agency Business ->
<DS:SIMG.FSL_REG_ARTEN>Define Payment Types</>).
Otherwise, contact your system administrator.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.