The piston of a rear caliper with an electric parking brake cannot be wound back by hand: the caliper has to be put into service mode with a diagnostic tool. Where that was bypassed, the actuator of the caliper is damaged, and soon after a disc replacement the caliper needs replacing too.
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The other complaint that recurs on owner forums is the parking brake fault itself: an "electric parking brake faulty" message, sometimes together with faults in the related controllers (hill hold, hill start assist, stop-start). In that case it is not necessarily the brake that failed, but the sensor or the communication between the controllers.
What to check
- Standing still, apply and release the parking brake several times: it should work evenly and without a fault
- After switching the ignition on, read the whole dash: there should be no parking brake, hill hold or stop-start fault message
- Look at the rear discs and the pad thickness: uneven wear points to a seized caliper