Yes, I have followed the common troubleshooting steps for error 30 and disconnect/reconnect everything. Also after that, I took each component (speed sensor, gear shift, throttle, main cable harness) and replaced them with the ones from the working kit. Brake sensors not mounted.
Now I'm trying to find someone in my town with a Bafang kit willing to let me test my display on his kit - I don't know what other option I have, most of the people I've read about online, they fixed ERROR 30 either by reconnecting sensors or by replacing controller/display.