[auser1989]

If you're saying "since build #0502b/#0503" then I'd expect there to be no issues with #0501 but that doesn't sound like it's the case here. The GPU firmware updates are not persistent so testing a build before #0502b will be using older GPU firmware, in which case any issues you are seeing are older than #0502b, or specific to your environment (try the suggested power cycle).

The crash is occurring in VAAPI and specifically the Radeon video driver - can you post a sample HEVC video (or reproduce with a Kodi sample) so that someone else with similar hardware can confirm if they also have this issue? Knowing the first build with this issue started (including testing older Kodi 18 builds) would help pin down the problem component - it could be a Kodi change. 

I tried powering off the AMD box completely end retried - no change (thanks for the suggestion @asavah).

I tried reverting to build #0430, same problem - just to confirm.

I reloaded latest build - #0520, retried again with a small file - same problem.   ( problem file - http://ix.io/1JD3 )  ( crash log - http://ix.io/1JCX  ).
I disabled HEVC-VAAPI in settings/media and tried the problem file again, played ok (a few seconds of sound and video, as expected, before eof).  ( kodi log - http://ix.io/1JCY ).
The problem file was created by taking the first 50,000 bytes from the from the front of the full file with a hex editor - I can upload a bigger chunk if needed.

I tried with a sample file from the Kodi Sample link provided - this worked ok (H.265 HVEC Test 1.mkv (242M)  https://drive.google.com/  ),
so I went back to some of my older mkv x265 files, from before 2nd May, and they, too, seem to work ok.

I hope this helps, please let me know if I can provide more info.

For now, it seems that I can use my AMD media box with the HEVC-VAAPI option disabled to continue using it as my primary player (as my other machine is too underpowered to handle 1080p/x265 playback).

thanks for your support.