It may be a hard failure.
Is the unit a new install?
Is this problem new or longtime?
Have any new AC devices been added recently?
Does it happen under a very light load? What about a very heavy load?
I'd check battery voltage directly at the input to the unit. If good, I'd set the minimum voltage setting down to zero or disable battery protection and see what happens. If the unit can display DC voltage, I'd check to see what it reports. It should throw an error code, but I'd check.
Next, I'd fully disconnect the AC input wires and see what happens. There could be an electrical problem generating feedback and the unit is chasing itself.
Next, I'd load the heck out of it and see what happens. Then barely load it, just enough for it to come on. The goal is to see if it is overloading or not detecting a load and going off. It should throw an error code in either case, but I'd check. If I'd find that one load level causes a problem.and another doesn't, I'd vary the devices a bunch and see if ai could confirm. That the load level was the problem or isolate it to a single device or set of devices or type of devices.
I have access to an oscilloscope, so I'd look at the output waveform to see what is happening, if it is malformed or cutting in and out. I'd also look at the input, just to be sure it is DC without significant AC.
Then I'd listen at the unit to see if I could hear the output relay or contactor clicking. Then open it and measure voltage across it to see if it was being held open or breaking. Then I'd measure the output to see if it lines up. If possible, I'd measure them simultaneously with truly analog meters or a scope. The goalnis to see if output tracks control and if control is steady or breaking. I'd be looking for a bad output relay or contactor.
The goal is to see if there is a DC input issue, an AC input issue, and AC output load issue, a sensor input issue, a waveform issue, an output switching issue, or a control issue.
Once I'd isolated the problem to one of those concerns I'd looknat the associated attributes of the installation, settings, sources, syncs, etc, to see if the problem could be bypassed (to test or until repair/replacement), repaird, or reconfigured to solution.