I ran into something similar today when putting in my new P2000. What fixed it for me (on a Windows Server 2019 install) was using the current alpha of Plex server: Much faster (hopefully), smaller (mostly) and better PMS builds preview (1.15.0.550-20055f39b)
Initially also no decoding through the P2000, but after installing that version (use the latest one at the end of the forum) it worked perfectly.
