So, I think I have found the solution. This should also work on Server 2012 R2 Standard without the Essentials Role and the Essentials Media Pack, as I did not want to install these because they force me to setup a domain controller…
When executing the BDA hack from https://knowledge.zomers.eu/dvblogic/Pages/How-to-add-BDA-support-to-Windows-Server-2012.aspx, you have to select a language at start (this is supposed to be the language of the original OS where the script guy extracted the files from… in his case: en-US). I selected de-DE, giving me some errors in the script because the zip file does not come with de-DE files. The necessary files from the script\files directory were successfully copied to the right place, but the localization files for my de-DE OS were missing, resulting in the tuner not working in WinTV and Plex. No idea why DVBViewer worked anyway…
What I did now was to extract the missing de-DE .ax.mui files (kstvtune.ax.mui, kswdmcap.ax.mui, ksxbar.ax.mui, MSVidCtl.dll.mui) from my german Windows 10x64 installation (from Windows\system32\de-DE and Windows\syswow64\de-DE) and copy it to my Server 2012R2 x64 german into the corresponding directories. Then I ran the script again. Now, I think the registering of the .ax files went fine since all to-be-copied files were already in place.
After that and several reboots, WinTV detected the tuner and was able to find channels (and play them!) and the Plex DVR setup wizard is now scanning for channels since 5 minutes. The LED on my WinTV DualHD is lightning also
