In 2019, Clark was embroiled in controversy surrounding his 2011 report, commissioned by the head of the Hohenzollern family, Georg Friedrich, Prince of Prussia, on the Hohenzollern family's relations with the Nazis. The report was in support of the family's claims for compensation under a 1994 German law allowing restitution for the loss of property confiscated by the German Democratic Republic on condition that the claimants or their ancestors had not "given substantial support" to the National Socialist or East German Communist regimes. Clark acknowledged that expressions of support for the Nazis had been made by the last Kaiser's eldest son, Wilhelm, the most senior member of the former dynasty in Germany in the 1920s and 1930s and the owner of the Hohenzollern properties. But his report concluded that Wilhelm was "one of the politically most reserved and least compromised persons" among the aristocratic Nazi collaborators, and that he was simply too marginal a figure to have been able to give "significant support" to Hitler – a position that supported the Hohenzollerns' claims.