Thank you Chris.
Actually, I'm a bit astonished, because what I was able to reproduce several times, doesn't happen anymore.
In a nutshell, what happened was a loop (program "not responding" in the Task Manager) once I've inserted in the master page the text box with the page number. The loop started as soon as I tried to change the fonts inside the text box or the orientation of the box itself. After then, other strange behaviors started also in the copy/paste actions with text boxes in the body of the document (only partially text copied, then loop...).
So, I've frozen the "bad" doc and started a new document without page number on the Master page. Everything was working fine.
Now, that I was going back to the "bad" doc to reproduce the loop and tell you something more, (un)fortunately the loop is disappered (Morphy teaches...).
If you want I can send you the (ex)"bad" doc...
Sorry.
Many thanks