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After some trials (rolling back the RADEON driver till December 2016...) I followed JIm Simon indication trying an "Unofficial Premiere Troubleshooting Guide" cleaning all plug-ins, preferences and cache.
So It worked after resetting Premiere (holding down Shift+ALT while Premiere Pro starts up and until the Welcome screen)..
The source of the problem is still unknown due I did not installed new plug-ins in the last months... but now it is OK and I have the real time preview again.
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