Uncaught InternalError: BLE task completed that wasn't scheduled (3/4)

then it should be 0x2A37 not "0x2A37"

I thought that might be the issue so I looked at the code, and the string should have worked (https://github.com/espruino/Espruino/blo­b/master/libs/bluetooth/bluetooth_utils.­c#L140)

NRF Connect is writing to descriptors not characteristics.

Yes, that's what c.startNotifications() should be doing under the hood. It finds the descriptor and writes to it to enable the notification.

Are you still getting randomly disconnected? It's possible it is because Espruino and the device can't agree on a connection interval. NRF.connect accepts a second argument of options which corresponds to: http://www.espruino.com/Reference#l_Blue­toothRemoteGATTServer_connect

You could try setting minInterval to 7.5 and maxInterval to 4000 to give it the max leeway in negotiating speed?

Not sure what else to suggest without having one here to play with I'm afraid. If you're feeling like diving in then I have an nRF52DK then Nordic have some tools to let you use it with Wireshark to snoop on Bluetooth LE traffic though