Thanks for your answers and suggestions how I could fix the problem. Unfortunately there is a bug in the current version of Plex, which isn’t able to detect the hw transcoding in Windows. This youtuber is a Plex specialist and described the same issue. (https://youtu.be/o82NWFnoDZw?t=2m45s)
I found a workaround to run Plex on Ubuntu 18.04. The hw transcoding is working without any problems or limitations and the performance is breathtaking. I’m able to transcode 8 streams simultaneously.
