I have similar experience on a 350 SLX, though the submerged platform is below the fixed platform rather than a part of it on the 400 SLX. On the 350 there are magnetic sensors which cut off the engines via the lanyard cut off circuit if they do not sense the swim platform in the up position. At the helm position, under the throttles, there is a covered 'platform override' toggle switch which will bypass this and you can then start/run the engines but know where the swim platform is and only run slowly or you will damage something.
For me the root cause is low fluid/a leak in the hydraulic circuit that drives the platform. With lack of pressure the platform can drop slightly, not visibly, but enough that the magnets dont see the platform and cause the cut out.
You will find there are three Mercury trim pumps in the engine compartment. One for each engine and one for the platform. In the 350 the swim platform one is aft of the engine one on the port side. I would check the fluid here, top if off and run the platform a few times to bleed any air out of the circuit.
If its not that its probably a warranty call.
