I'm in the middle of a trip with a similar issue. Mine comes up once or twice in a long day of driving. Have been unable to find a dealer or mobile tech I can coordinate with to check it out. Everyone is very booked up. In my case, most of the time it is just the message, but once I did try to stop and had no trailer brakes for a brief bit, then they kicked in (not full, just normal...but still scary...)
I'm guessing a bad ground somewhere. A tech in a campground (who couldn't actually fit me in) advised to check the trailer plug - make sure contacts were not dirty. It has two prongs for each of the separate pins. A screwdriver can be used to slightly bend one of the pins inward, to provide a tighter connection. That did not help mine, but it may yours.
Underneath, the cord goes to an electrical box. Inside mine was a bunch of white wires all bundled together with a big wire nut. Mine was not fully tightened. We'll see if that helps the problem. While under there, you can also see where additional white wires are bolted directly to the frame to make it grounded. Check that everything is in good shape and very tight.
If my problem continues, all I can think is to maybe start pulling wheels to check how well the main brake magnets are grounded....not a thing I want to do in a campground.....
