Had an E mail from the club I bought the motor glider from, seems to agree with ChickenHouses post
"sorry for the late answer. We are also astonished. There is something pending between the Bundesnetzagentur and the Deutscher Aeroclub. The are fighting already some years because oft he sense and entitlement of this invoices.
The statutory period ends within 4 years, therefore they were forced to invoice not to lose the claim.
We got the same invoice for all of our gliders (about 700.- Euro) and discuss, If we can lodge a protest or if we have to pay for it.
We have a meeting on Saturday, afterwards I will contact you and let you now, what we do.
The invoice is due to first of december, so we have still time."