Marvin might be onto it.
I got 2.33:1 as your ratio.
With 900rpm, you are at around 2400rpms. I assume it cuts out at around 2200 or pto 825rpms, with a centripetal clutch. so it might just be derated from woodgas by 10%.
however, according to:
3. Line fault protection: The Inverter will auto stop when load abnormal or short current.
I didn’t pay attention to what load you put on it.
I would try belt dressing if you haven’t put any on yet. or rosin or something.
I would think about wrapping the flywheel in neoprene or spray on adhesive/sticky stuff.
Also, I would put a lever under the genset and lift that up, when you first start it up to get the flywheel up to speed.then engage it by dropping it back down. I assume you are tensioning those belts with the weight of the genset. (if not the belts may have stretched a bit as they got hot, but I am sure you are aware of that.) Then you let it get back up to speed before applying any load.