epub to pdf conversion: blank page inserted before any section

[Nicoo]

Hi All,
new thread to beter define the issue I described previously.
It seems that Calibre inserts a blank page before any chapter or section defined in the spine section of the "content".opf file. Checking "none" in the chapter mark section of the structure detection does not change anything, still useless blankpages are inserted in the pdf file. I tried most of the possible settings with no success.
Did anyone find a way avoiding this?
Thanks and best regards.
Nico