Interesting. I had a booking in Italy and was similarly unimpressed. The originally rented car was unavailable as the driver had an accident when driving the car over... Gemma has the details already.

My issue got cleared up to my satisfaction however by calling the Sixt hotline who was very responsive and escalated the issue appropriately.

I have found in the past that the franchise model is unfortunately suffering from franchise partners trying to offer less than the promised/expected service. A phone call to the central hotline of the company explaining the issue has generally helped. It did in this case as well as with problems I had with Mercedes-Benz and others.

Give it a shot the next time.