Though suspicious of the BIRD, the first thing I'd check after so recent a battery change is the chassis battery post connections. Like you, I'm dim about how all these things fit together and regularly have to resort to the manual, which often isn't fully helpful. If your genset runs off the chassis rather than house bats, then perhaps drawing what was left of a high resistance current (due to corroded or loose cables) when you engaged the ignition, took sustenance power from the genset, causing it to stop.
I'm purely guessing, and am sure others here will chime in with more educated ideas. But meanwhile, check the chassis battery post connections as a first resort, and use a meter if you have one to check their state of charge while plugged in, with the genset on only, with the engine running only, and after having sat with no charge source for an hour. Those numbers may be helpful to fellow advisors here.
You might also locate and check the chassis ground point. Your ignition is probably okay since the panel lights up and you hear the starter solenoid engage.
Joel