R/3->XI Idoc processing error.

We are trying to send an IDoc from R/3 to XI (PI). The IDoc has status 03 (IDoc sent to R/3 System or external program, Data passed to port OK). But we do have the following message which is probably returned by XI:

No service for system SAP<sys-id>, client <client> in Integr

Does anyone have an idea what to do and where?

Is it something in R/3 (guess not since status seems to be OK), or any configuration to be done in XI?