thx a lot for your hints and thoughts.
I checked a bonding of the Davits. No bonding to see + more then 5K ohm between bonding and davits, no bonding in the wiring diagram. Motors are with 24V (mostly 26V).
I checked the residual current protection switches. They are working/ ok - and 220V is obviously not relevant.
Inverters are turned off most off the time (only on for hot water, cooking, with generator for washing machine, dryer or dishwasher) and both ALWAYS turned off when using the davits.
No unusual corrosion so far.
No electrical leaks (since appr. 1,5 years). Detector is working.
That is why it is a riddle (at least for me) and interesting to solve it.
A54 #119 Lady Charlyette, currently Eleuthera