Thank you for posting.
Are you able to reproduce this issue when printing from a different computer? What about in a brand new database?
Well, since I couldn't print from a different computer I tried with a brand new database but the result didn't change.
So I started from scratch testing all the parameter combinations and suddenly it worked. The trick is to set five different parameters in a proper way (you can see a couple of screenshot linked below): making this, the printout starts without any request. The pity thing is that these parameters aren't retained not only between working session but even between consecutive printouts :( and must be rechecked each time.
Ok, so far the customer is using the program and doesn't complain any more.
I have another issue but I will open a new thread for it.