I wanted to let everyone know that I was able to fix the problem even though I will have to open another thread to fix the cause of the problem. The cause of not being able to enter the Debian user desktop was that the / partition was too full! I had installed Debian with 2 partitions (/ + home, and obviously swap) It was enough to remove an app and "magically" on reboot I was able to re-enter the user desktop.
Thanks for suggestion and only thank to be able to enter to "root desktop" made me able to remove the app and solve the problem.
Francesco