BancoSolar

Triple check that the battery voltage isn't collapsing. When the DC-AC converter starts, there is a sudden surge of power draw; if you are testing with a tiny battery or weak power supply, it may be that the battery voltage collapses with the load. The newness of the board has to count for something.
I wonder if it's old stock, so that capacitors could have dried up even though the board has seen no active use. Perhaps check a few chips and/or the PCB for date codes.
Check U10 pin 7 (Vcc), and U10 pin 1 (compensate input, pulled low to reduce output). U10 pin 6 (output), Q36 drain. From the results of these measurements, you should be able to plan another series of tests before components come off. Assuming that the battery is OK, I would suspect the control circuit pulling the compensate input low; if so, check U7 and surrounding components.

@BritishRacingGreen has reported that a lack of -12 V power affects the measurement op-amps, and the battery voltage is the most important of these. It's because the measurement op-amps use +12 V and -12 V rails; the type of inexpensive op-amps used can't operate right down to zero volts input. However, usually the lack of -12 V results in measuring too high of a battery voltage. I guess there must be some mechanism whereby the measurement can end up zero.

Morning All Axpert Cards"Voltronic has an onboard 30A relay to Do the same Function as the external Anti Island relay that needs to be installed on the outside of the SunSynk and Deya Inverters The Main test is to add battery voltage and test between output neutral and ground for resistance Pls remove The incoming cables before test The test indicates a dry contact between output neutral and ground 

Morning All Axpert Cards"Voltronic has an onboard 30A relay to Do the same Function as the external Anti Island relay that needs to be installed on the outside of the SunSynk and Deya Inverters The Main test is to add battery voltage and test between output neutral and ground for resistance Pls remove The incoming cables before test The test indicates a dry contact between output neutral and ground 

Morning All Axpert Cards"Voltronic has an onboard 30A relay to Do the same Function as the external Anti Island relay that needs to be installed on the outside of the SunSynk and Deya Inverters The Main test is to add battery voltage and test between output neutral and ground for resistance Pls remove The incoming cables before test The test indicates a dry contact between output neutral and ground