IDOC-Adapter (HCM to PI) Sending Problem

Hi PI-gurus!

We're trying to implement our first IDOC scenario on PI which for testing should look like this:

HR Backend --> IDOC (Messagetype HRMD_ABA) --> PI --> SFTP/FILE

On PI we tried to use the normal IDOC-adapter and the IDOC_AAE adapter.

Both have the same result:

The IDOC are not sent, but create an error in SM58 / tRFC-queue of the backend.

We got two types of errors:

Mostly, it's this one: Use IDOC Adapter for FM IDOC_INBOUND_ASYNCHRONOUS. --> What does this mean?

Sometimes we get this: Authentication failed.

We are using a SAP ECC 6.0 backend and SAP PI 7.3 with dual stack.

I guess, some configuration is wrong. Does anyone have an idea?

I also wonder about the following questions:

1. Can we use the IDOC_AAE adapter, when having a dual stack system?
2. What are the RFC-Parameters in the IDOC-Sender for? Is this a back connection to the backend or a connection to the ABAP stack of PI?
3. Do we need to configure anything in NWA if we choose "Manual" option?
4. Are there any good tutorials?