To the OP. Glad you resolved the issue.
I had the same issue with my F450 when I got my new MS. As stated earlier the IBC knows the trailer is there, but when pressing the brake pedal it sense the trailer tail lights for a load. The LED's do not provide enough of a load so it thinks the trailer has disconnected. I had the same message followed by Left Directional Fault and then Right Directional Fault.
For anyone else having the issue I added an adapter to the trailer umbilical cord. The info is:
It cost me about 20 bucks at a local trailer hitch shop. You can alos get it at Amazon.