After a lot of work and a little flu I found some time to take care of the MC21 to 25 upgrade again, finally
As Roderick has suggested, a clean install and a restore from 21 has helped a lot - still some flaws in expressions but after a little thinking easy to fix. No more math for sample rates, that's cool. And I'm very happy for the subtitle fix with Intel620, that was the main reason for the upgrade - and the WDM driver problem after some Win10 upgrade.
But.
Still not able to use the WDM driver - it shows up as a playback device named 'JRiver Media Center 21' with controller JRiver MC25. Doing a test directly from the sound control, I can see the level meter reacting but no output on any device. No surprise.
So I removed the WDM driver from 25 and from 21, made a reboot and than uninstalled 21 completely - incl. reg-keys and library (already had an external backup), another reboot, then I activated WDM in MC25 again - same result.
Then I checked the eventlog and found a lot off errors (informational ones) connected to MC21 (and some of 22, 23...all fully uninstalled)...weird. Further checking and analysis led me to a lot of orphaned folders and reg-keys and even the jriver service from 21 was still showing up in the list, not functional of course. Looks like the uninstall routine is not very successful, at least on my system (W10pro64 - no additional AntiWhatever).
Next I cleaned up as much as I could without killing my system but still, no success and still the same strange '21 with 25' entry in the sound control panel. At least, service is gone and no more EventLog entries for MC21.
I also had some hope it will fix the issue with another error I found in the log, 'JRtools.dll_unloaded', leading to the packetinstaller.exe in the MC25 install directory. No luck. I checked the dll, signature looks ok and valid until 2029...was a bit surprised it's from Symantec, but hey...
As I have a little experience in system engineering (that's how I pay my rent) I used some tools to dive deeper - but I cannot find anything left from MC21 or any other install than MC25.
WDM is an extremely nice feature I don't want to miss but I also have no intention to do a fresh install of the whole system. There is already a lot of work I spent for some individualization and stuff, so...Frankly, I'm not used to not being able to fix bugs either.
Still working with the eval. version but only a few days left now...Next I would remove the MC25 completely as well (I did some re-installs over the existing one already), clean the system from residues and then put 25 back on...but maybe someone has concrete hints on which screw I could turn?
Many thanks!