SOLVED: Arc A380 transcoding suddently stopped working

[Host-in-the-Shell]

Did you by any chance update your bios to a new version recently? I had this issue pop up for me after updating mine and what happened was that the update changed many options to default that I required to be able to transcode with the Arc380. That varies from case to case, but for me it disabled 4g decoding, resizable bar, ASPM, and reenabled CSM support. After changing the aforementioned settings, everything went back to normal.

I'm not sure if Ubuntu does automatic firmware/bios updates or if you happened to perform one yourself, but just in case, you should check that your bios settings haven't been changed.