Adult Mystery: Nunnery has very valuable medieval tapestry, but cannot sell or display it because its imagery is highly pornographic. Written in German. Read in 1990s.

[Monika]

I read this in the nineties, and I think it must originally have been written in German. It is set in the present, and an important part is played by a nunnery in great financial trouble. They could solve all of their problems by selling a hugely valuable medieval tapestry they have owned through the ages, but the problem is that it depicts Tristan and Isolde under the influence of a love potion they have just drunk, and is extremely sexually explicit.
The mother superior consults a local art historian who specialises in medieval art, and whose work is all focused on how the main characteristic of that period is its fabulous restraint, and obviously this blows that all out of the water. I remember once scene specifically, where he is looking at the tapestry for the first time, completely bowled over, and mutters to himself "It just goes to show you're never too old to learn", to which the nun (thinking he means sexual techniques) replies "That really wasn't why I wanted to show it to you"... Eventually, the mother superior, who is also a shrewd businesswoman, decides to put the tapestry on display in the nunnery, rather than selling or donating it to some archive, charging a substantial entry fee, thus putting their finances back on a solid basis.
I think some sort of murder or something was likely also involved, but I cannot remember any details beyond this.
Ring any bells?
Thanks in advance!