This is a constant problem at our place with the Cartwright trailers we use.
Haldex,who supply the EBS system on these particular trailers,were,at one point,coming out regularly to re-set the system,after Renault discounted the fact it was the Premiums that were the cause of the problem.
But what we discovered,was that if you left the engine running when connecting the trailer,it was tripping the EBS system to show it was faulty.
The solution was to connect the trailer with the engine AND ignition switched off,then once connected,turn on the ignition,let the unit and trailer do their respective checks,fire the engine up,and all was hunky dory.
Ken.