Seeburger AS2: Error on send - illegal MDN - content-type not supported

Hi,

I build up a connection to a partner to send ORDRSP via AS2.

Unfortunately I'm receiving an error in the Seeburger Workbench:

Received illegal MDN - content-type not supported!

I set up the content-type: application/EDI-X12. I have already tested different content types, but I always get the error. The whole conficuration is similar to our other partners.

Do you know, what this error means and give me an advice please.

Thanks a lot

Frank