To analyse how the SAP adapter retrieves this list of IDocs, we traced the RFC communication between SAP and BizTalk (transaction ST05*) and found out that the SAP .net Connector uses the BAPIs RFC_SYSTEM_INFO and IDOCTYPES_FOR_MESTYPE_READ to get a list of IDocs which are released:
Using transaction SE37, we (thanks, Rüdiger!) could analyse the BAPI IDOCTYPES_FOR_MESTYPE_READ and see that it uses the table EDIMSG to get the IDocs. And their we saw that the BAPI doesn't call for "message types" instead of "basic types". And, guess what, the message type had a different name then the basic type (which makes perfectly sense) and, thus, couldn't be found.
* Tipp: When logging to SAP to trace, you have to use the same user that you use inside the Send Port / Receive Location.
