Diagnosis
The request IDoc could not be sent to the source system using RFC.

System Response
There is an IDoc in the Warehouse outbox that did not arrive in the ALEinbox of the source system.

Further analysis:
Check the TRFC log.
You can access this log using the wizard or menu path "Environment ->Transact. RFC -> In Data Warehouse".

Error handling:
In particular, check the authorizations for the background user in BI.