Using ffmpeg to stream to or from Blackmagic Decklink Card
I know this is pretty old BUT as far as I know ffmpeg and Blackmagic Gear, you will have to set your input options according to the Stream it gets from the H-264 encoder.

So you need to tell ffmpeg that it gets H-264 encoded video and AAC encoded audio - then it should work.

This is not an issue with the DeckLink cards as they output raw video data.
You might give it a shot...