Just to close the issue: I've set gpu_mem at 320 Mb via raspi-config and now everything works fine! Thank you very much!

@BuZz said in Kodi 17.3 Crashes when detecting x265 files:

@maxbeanz they do the same thing. You must have chosen not to update system packages.

I have updated my 4.x Retropie image at least twice since installing. I chose 'Update All Installed packages' and when it asked if I also want to update the dependencies etc. I chose yes. I never saw it say anything about upgrading Raspian Kernal during the process. Since Kodi 17 it's never been able to play x265 files until I chose 'Raspian Tools / Upgrade Raspian Packages'. Maybe the script isn't fully working ? Can @Rascas confirm this ? Anyway, Kodi can play the files now.