While it takes some effort for German citizens to legally buy and carry loaded firearms, it’s entirely prohibited to own any weapons of war. According to Insider, the man violated the Weapons Control Act by possessing military-grade weapons. His lawyers, however, argued that because most of his collection was non-functioning, he could not be charged. ^(source)^
TL;DR He lost the whole collection because of all the stuff like the working machineguns and flak ammunition he had laying around, in addition to the tank.