Sorry, I have no idea how to; it just seems that the back of the CHAdeMO connector would be the place most likely to be affected by road salt, as it's near the front of the vehicle, and there would be gaps between the wires and the housing where the salt could work its way in.
But as you say, try other things first, as it seems that it's more likely a signalling issue. Now that I think about it, isolation signalling sounds like something possibly internal to the on-board charger, where it tests isolation of the battery from the chassis. This entails isolated op-amps and transmission of the resultant measurement to some ECU somewhere. Maybe the isolation testing is actually inside the battery case; it might even convert the measurements to digital values and transmit them over the CAN bus or other means to the appropriate ECU. It seems like that conversion or transmission is failing.