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It sounds like the common failure of the gears. It's not a major repair and there are lots of threads about it. It's pretty easy to remove the speedometer and there are a number of good shops who routinely fix them, my personal go-to is Palo Alto Speedometer. I'd suggest documenting the date and mileage of the failure for future reference if you ever plan on selling the car. At this age the gears are pretty brittle and repeated resetting the trip odometer, particularly when the car is moving, is often all it takes to make them fail.
If the speedo works and the odo and trip don't, it is almost certainly a single gear in the assembly. Common problem. A careful DIY can fix it for the price of a $30 plastic gear.
In the past I have read that the problem was that Porsche used an incompatible grease which deteriorated the gear over time, so it is a time thing more than a wear thing. So your ultra low mileage car makes sense here.