NonCredibleDefense

It also restricts ownership of weapons of war.

Since the tank was ancient, it did not qualify as a weapon of war anymore and rather as a museum artifact. But, from another article:

This “Panther” was not the only military device there that investigators suspected was illegal: the operation also uncovered a torpedo, a 5-centimetre caliber mortar and an 8.8-centimetre anti-aircraft gun. The resident of the house also owned machine and assault rifles, semi-automatic and fully automatic pistols and more than 1000 rounds of ammunition.

The 84-year-old was ultimately only convicted of possession of a weapon of war in the case of a machine gun barrel and two cartridges.

That's of course only the violation of the War Weapon Control Act. He was also found guilty of illegally owning "regular" weapons and explosives in the remaining cases.