Thanks for your extremely fast reply.
I read the manual and the 3 reasons for "freezing" are not valid in my case. What I mean by crashing is simply the program closes and disappears immediately, I need to launch it again if I want to use it (and it launches with no problem).
I use a utility called Soluto
which generates a message whenever a prog is crashing. I attached the one "informing" me about the crash which happened again just now (see below). By the way, when I click on the "More Info" the utility states that this problem is new (for the prog) and it can't provide info or solution.
Your prog, Calibre, is awesome and I hope that you'll be able to help. If you want me to attach file for debugging let me know which ones you need to see.