There's something fishy in your water heater's wiring/grounding - that's why the fault stayed even after the heater change.
It could also be a leak somewhere in the hot water pipeline downstream from the heater. (or an inadvertent connection to a neighbor's hot water plumbing ??).
You must have very limited points of use for hot water - Kitchen sink, wash basin and shower are most likely points ... Why don't you forget about the existing heater, hot water plumbing and heater wiring and run something like
this from a separate high current socket?
This type of tank-less instant electric water heater will help you cut out the problem rightaway. If not feasible, please go through 1. hot water plumbing for hot water leak 2. heater wiring for leaks and grounding issues. You must get an answer with this. Good luck!