Yes, Horst Tappert was a member of the Waffen SS. There is no doubt of that. What historian Jan Erik Schulte may be referring to is whether he might have been "pressured or coerced" into joining the SS.
Tappert never mentioned the fact of his SS membership in public. However, unlike German author and Literature Nobel winner Günter Grass, he did not pretend to be a brave fighter
against fascism while keeping quiet about his war record.
Earlier this month, Germany public TV broadcaster ZDF announced that it would not broadcast any more reruns of "Derrick", the detective series that had made Tappert famous. In the Netherlands, "public television channel Omroep MAX said it had scrapped plans to show around 20 episodes of Derrick from July." (BBC).
Tappert was an actor, but his character Inspector Derrick was the creature of Herbert
Reinecker, Germany's most prolific TV screenwriter for decades. Reinecker had been a
fervent Nazi from the start. He, too, was in the SS (in the same Totenkopf division as
Tappert), but he also had a career as an influential propagandist, beating the drums of war from 1935 when he joined the Reich propaganda ministry right up until the last month of fighting in 1945.
The link in the paragraph above goes to a a German-language article in Die Welt from 2011 reviewing a book analyzing Reinecker's war and post-war careers. The author of the book concludes that Reinecker -- who never made a secret of having been in the SS -- remained an apologist for Nazi crimes after WW II, without being explicit about it. Episode after episode of "Derrick" and other TV shows written by Reinecker features protagonists who run afoul of the law with the best intentions.
The tragic dimension in crime was also explored by other writers such as Georges Simenon,
creator of Inspector Maigret. But Reinecker's writing frequently was barely more than a
coded apologetics to vicariously absolve Nazis in particular, and Germany as a nation, of
historical guilt. Many episodes condemn the "dredging up of the past" -- again, without
explicitly referencing Nazism or WW II, but the underlying message was nonetheless
appreciated by the viewership, in a wordless kind of way.
It is unclear whether Mr. Tappert understood this. He was not known for being an
intellectual. His appeal as an actor came from an understated virility and his ability to
credibly represent the power of the state to catch criminals, a comforting notion to people all over the world. Yet even Tappert is said to have recoiled at playing Derrick in what was to have been the final episode of the series.
According to a 1999 report in Berliner Zeitung, that episode was to feature a
photograph of the infamous ramp in the Auschwitz death camp in the service of an abstruse
plot about a Ph.D. madman who induces young people to suicide by showing them scenes of great horror. Apparently, such a cynical use of the Holocaust was too much for Tappert and he refused; the episode was never produced.