Update:
Finally I received all the components.
I have replaced all 4 UC3825 Chip on the controller board. On the inverter boards I replaced a pair of TC4429 with Max4429, Each inverter had a pair, I ended up replacing for all the inverters, so 8 in total. Both the 33ohm resistor was replaced for the soft start circuit.
After securing back everything, I turned on the power supply. For the first time ever, the relay clicked and 300V went to all the inverters. The Control Error Indicator turned off and the power supply came to life. Full voltage of 60V was measured and it was spot on. I was happy for a brief moment, till I started seeing the output voltage started dropping back to zero.
Powering up again the PSU behavior is same, I tried isolating the issue by removing the control cable of each inverter. When only one inverter (any of the 4) is operated alone, the output voltage seems stable. But as I add more inverter the PSU output shuts down slowly.
Another observation I made is before the output dies, the output voltage fluctuates, example when PSU is set to 50V, the voltage fluctuates from 48-52V before shutting down.
Will keep looking tomorrow, my suspect now is at regulation and output rectification section, at least its good progress