Usually I’ll see that type of error when I send in a Flat File and it’s expecting XML. You might want to double check your Receive Pipeline. Make sure it is set correctly. You will need a custom pipeline using the Flat File Disassemblier that’s using your Flat File Schema.
Oh, also make sure your schema is defined as a Flat File Schema. I have some samples and a video that covers more advanced Flat Files – but they might help to look at.