But the biggest boost to Nolde’s image came with the 1968 publication of Siegfried Lenz’s best-selling novel, “The German Lesson,” which is still required reading in German schools today. It centers on an artist called Max Ludwig Nansen, widely understood to be based on Nolde, who produces a series he calls “invisible pictures” when he is banned from painting by the Nazis.

Image Nolde in Munich in 1937. Credit... Helga Fietz, via Nolde Stiftung Seebüll

Particularly among the generation that grew up in the 1960s, embittered with their parents for enabling dictatorship and war, Lenz’s novel elevated Nolde’s “Unpainted Pictures” to symbols of heroic artistic resistance against a tyrannical regime. The foundation’s museum was flooded with visitors.

Until 2013, the Nolde Foundation “remained wedded to the Nolde legend, the story that had been selling so well,” said Bernhard Fulda, a Cambridge historian and one of the independent experts who examined the archive. “In a way, the foundation had started to believe its own propaganda.”

The other expert, Aya Soika, a professor of art history at Bard College Berlin, recalled writing an email to the foundation in 2003, asking whether there were letters in the archive referring to an ugly incident in 1933, when Nolde incorrectly reported the artist Max Pechstein as a Jew to the Propaganda Ministry.

The foundation never responded to Ms. Soika’s request. But when she finally gained access to the archive in 2013, she discovered a file labeled “E.N. and Jews.” The document at the top was her unanswered email.