Hi Ajnsan,
thanks for your considerations, I think it is due to the excessive heat of the nozzle as you assumed.
at the beginning I printed 2 pieces at a time (2 failed attempts), then 1 at a time 1 failed attempt.
I reset the z-axis and changed nozzle (1 successful attempt and 1 failed)
Then I reset the z-axis and 1 successful attempt.
They are quite elaborate and large pieces and therefore I have not printed them anymore (4 failed pieces)
now I have printed 2 pieces of 15 cm and everything went well (for superstition I have always done the z-axis in Home) 
I will keep you informed if I have news on this matter
Thanks
Best regards
Luca