The same problem i got on my Axpert VMIII 5KW 48V SYSTEM,once i switch "On" our water heater it will gives ERROR 55 or HIGH DC ON INVERTER OUTPUT,but this problem has been resolved by installing additional RUN CAPACITOR 50uf 450v connected parallel on MAIN CIRCUIT BREAKER OF WATER HEATER😊
I have the same problem once we use our water heater this fault will occured on my inverter but i am planning to install additional RUN CAPACITOR connected in parallel on the water heater supply breaker.