I found the responsible… Me, 3 years ago:
It was burried within the Preference.xml file, in the middle of others options. Thanks ChuckPa for spotting its activation.
Why this 3 years old workaround was working until now and not anymore : no idea.
But by removing this old workaround and the symlink workaround from my 1st post (which was a workaround of the first workaround in the end 
) everything is back to normal, hw transcoding working, no workarounds involved 
