Hello again,
Just to give you the full picture of the problems I experience after updating to calibre 6.1 64 bits :
1/ kepub output extension does not work properly (Error "No module named 'PyQt4'"). This is very nicely solved thanks to davidfor's kepub output beta version
2/ Calibre is unable to properly launch the E-book viewer 64bit when clicking on a book in calibre (details of the error log attached), although the E-book viewer is properly installed and running when launched directly from the installation folder.

I have uninstalled and re-installed Calibre, but this did not help...
Is there any hint of a new version of calibre 64 bits fixing this last issue ?
Thank you !