It was a faulty battery

Just an afterthought.

The battery did turn out to be the problem.

It tested ok on the bench, but when the Gazelle technicians put it in another bike they encountered exactly the problem I had. It registered full charge but did not supply any power to the motor.

They downloaded new software and that fixed the problem.

The battery also required a new backplate (the part with the LEDs).

Not sure if the two problems were related.

I cannot fault the service I got from Gazelle. They were courteous, patient and genuinely pursued a resolution to the problem.

Just difficult to do it from 400 km away.

One thing I was a bit surprised at was the warranty on the battery. It was a set time (2 years), not a set number of discharges.