It sounds like a combination of inrush on the motor, plus voltage drop from the 50' extension cable.
Motor loads usually have very large inrush current demands during start up, once the motor is running, the load demands shift down to the "running" requirements, but getting them started is a huge load. These are normally listed as "surge" demand and it can easily by 2x, 3x the running amperage.
You won't be able to see the peak inrush without a very good clamp meter or oscope. It's usually happening just too fast to be captured on lower end meters.
Transformerless inverters do not handle those inrush demands as well as transformer models. Transformers have a little more reserve capacity and can smooth out the dips while the surge is occuring.
You've also got a very long extension cord which is inducing voltage drop on the circuit. As the volts drop, the amperage requirements go up, which is adding even more load demand on the circuit and inverter.
I don't know what the Bp Battery Disconnected error really means, but if the battery BMS is disconnecting under the load, then it's reading it either as a short circuit or over current and it turning off to protect itself.
