I have done a few more testing. The result is: I don't get it to work.
After I reset the MS-DRM as admin, did what has been proposed (download the file from Performer, play in performer, play in Windows Media Player, play a file in Performer, download a new file) everything works fine. (All Anti-Virus Software is paused at this stage). After the file is downloaded I see "complete" in the status column of the download view. So far so good.
Then I close MC19, open it again, open Performer, download a file - also and still working.
Then I simulate next day: restart the PC open MC19, open Performer, download a file - not working anymore.
This is not what I think it should be. (By the way: I can't be the only person with this kind of problem)
So, if I do not get e proposal for a good solution from JRiver in the next few days, I will cancel my subscription of Performer. I still love MC19. It seems that Performer is not well maintained over time ... since it worked in older versions without problems.