I am having same problem error code 51 in my inverter and it just got damaged. It happened when i changed charging current paramters Total PV + Utility from 20A to 30A and explicit utility charging current from 2A to 20A. There was very nominal load (160watt) runing on inverter. Both utility and pv where charging.
Inverter Ratings (5KVA-48-PAR-A):
Rated Current: 60A
Voc: 145V
My pv Voc is max 125V. ( 2 strings of 4 panels each) each panel is Voc 31. So 31x4=124v which should be fine for a pvmax 145v inverter.
I am not sure why this happened because i was only runing a load of 160watt at that time. I am sure it happened on changing charging parameters.