X3 audio engine drop out after win10 upgrade

Interesting (minor) problem here with workaround available, but I'd still like to understand what could be causing it:
 
Our live system runs really light Sonar projects (no FX, click track, a few audio tracks, plus MIDI to control external gear). We used X3 so far and wanted to stick to it as it has proven realible in many shows ...
 
After upgrading the DAW to win10pro (in place upgrade) we have the strange problems that few (3-4) projects stop after about 5 sec (i.e. during the first 3 bars) with audio engine drop out although all there is going on are MIDI signals being send and a  1-2-1-2-3-4 count in via individual audio files.
 
What makes it strange is that all live project are identical up to this point. All work apart from 3-4. When I bounce the count in to a single track and remove the individual WAV files for 1,2,3,4 it works ... it also works in Platinum (without boucing, no change required)...
 
So I got 2 workarounds (bouncing, or going for Platinum) but I would still figure what could be causing this ... it's not the 64 bit engine, and none of the audio related preference settings (I think I tried all ...) and changing ASIO buffer does not help ... don't know about read buffers (but projects are very light and run on a SSD) ...
 
Any thought welcome!