My easysolar 5000 is two months old and has started tripping out with load of 2000 watt.
Battery pylon tech 3500. Four year old.
Would one suspect the battery or the inverter.
At first, I would check the DC connections. When they are loose or faulty, the multiplus could cut off under high loads.
If it is one battery on a 5kVa, i would start with the battery.
A US3500 has a continuous discharge current limit of 35A that roughly translates to 1500w load.