Per Crion's advice, I installed .net version 3.5 and that took care of it. Thanks.

It is a bad thing that the M installer didn't specify which .net version it wanted and just requested the latest version, which was already there.

It is a strange thing that several machines were ok, one was not. From the replies, it seems others have equally random experiences.

(Crion, I note you're from Stockholm. So am I, moved to the U.S. 35 years ago.)

Music room: 818v3, DSP8000SE, Roon/Intel NUC, microRendu/LPS-1 into Geek Pulse Xfi into Audeze LCD-3
HT: G61R, HD621, MS600, Oppo, Avantgarde Solo
Library: MS600, Bel Canto, Mark & Daniel
Other: SonicOrbiter, GeekOut, Audeze EL-8