Have you tried other players ?
Try other player like MPC-HC in order to check if Plex is the problem.
What is your OS and your CPU/GPU ?
Do you have a discrete card installed or Intel iGPU only ?
I have a NUC, NUC5i5RYK, so it's Intel graphics.
Windows 8.1 pro
I didn't see this issue with the older graphics driver, but I can't go back since the newer driver fixed a few issues.
Ah, yes I didn't pay attention to NUC.
So, you have a Broadwell Core i5-5250U with HD 6000 GPU.
Haven't noticed any weird decoding using my Haswell Core i7-4790/ HD 4600 and the same OS and drivers like you.
Everything is the same regarding DXVA.
Is it possible to upload somewhere one of those VC-1 encoded videos ?
The videos are my ripped blu-ray's and over 20GB a piece.
So I can't
I played the video's using Plex on my ipad and had no issues.
Just on my NUC...when DXVA was turned on...and with this latest driver.
After further research I've concluded that the problem lies with PLEX not using current version of ffmpeg.
Their copy of FFMPEG is old and prone to this issue.
They say they are working on upgrading ffmpeg but it will require a larger update to the whole package.
So it will take a while...which sucks.
Here's the posting