Hi Mainer,
well unfortunatly herein lies the problem with 'washing' an electric truck. Although covering the motors is a good idea it is also important to keep the control switchs and control panel covered as well. The only good solution for cleaning electronics is a fast drying low residue cleaner like contact cleaner. If you use anything else you then have to go through extra time to use compressed air to blow out the areas where the liquid gets into, and even knowing that the liquid will find its way into switches etc and it is very hard to dry those out.
Even though the control panel is enclosed in the rear counter weight behind a cover moisture and liquid can still get in there and cause alot of problems.
The only good way to dry one out is to use heat and fans and allow for the time it takes to dry out.

Now with that said...
The code -142 relates to the current sensor in the hydraulic control. If you look at the rear panel at both control cards there are 2 panels side by side, the one on the left is the drive control and the one on the right is the hydraulic control, then you can draw a line below both of them and you have the contactor panel area. Now looking at the hydraulic control panel there is a shunt bar assy that has green and yellow wires (which should be a twisted pair of wires). The error pertains to the green wire having a problem so check that wire for any breakage, trace it back to the control card and pull the main plug and check the terminals inside the plug and the controller socket pins as well. Blow it out with air also to make sure there is no moisture in it. The error is cause by it loosing it's ground briefly when it expects to see current through the motor circuit and it does not. There should be a large 350 Amp buss fuse inline with the hydraulic motor circuit, check that fuse. Don't rely on just looking at the element inside it, remove it and check it with a vom. But even looking at it you should be able to tell if it is blown out, it will be obvious.

As far as the contactors chattering i think you'll find when you get the lift dried out that should go away. While your blowing out other components make sure you blow out those contactors as well.