Can measuring back EMF spikes damage the oscilloscope?
Yes it can damage the scope or the probe or both.
A scope that is rated to handle 400V without damage should not be used to measure voltages over 400V.
A 10x probe will divide the voltage by ten, but then the probe must be rated to handle over 400V as well, or it might be damaged. And if the probe gets damaged, it can pass the full measured voltage to scope and it may again damage the scope.