The maddening thing about these charges is that you have to predict how long you will require to stay in the car park, these machines being the cheapest to install. Most in Germany and elsewhere are the type where you take a ticket on entry and insert it in a machine when you return and are told the cost which you put into the machine in the form of coins or notes and change can be given.
If you for example have a dental appointment you can find yourself with serious delays, you can't return to the car and are then fined, which is not your fault, and is unfair.
What I suggest you do is write and say why you were delayed in returning, they will probably counter that with some bye-law which puts them in the 'right', ignore that and write again - and again if necessary, and they will give up.
Having an expired ticket is not considered as bad as having non at all. You wont be taken to court.