Sounds like the same basic design of controller BT use on the old DC stuff in Europe.

I'd physically double check the traction fuse as I've seen issues with burnt fuses causing odd throttle issues even though a multimeter says the fuse is fine, I have also seen these type transistor panels burn transistors out.

Have a sniff of the panel, if it smells burnt you then need to investigate further.

There is only 4 self tapping screws on the back of the panel that holds the plastic top on - be warned that opening the unit normally voids any warranty / service exchange return in the future & mind the capacitors inside!

As I said before we don't get to see the Prime Mover stuff over here in UK but it sounds like it's a basic BT parts bin DC equipped machine.