I did not find the solution in the troubleshooting guide so I decided to post it.
Somehow I could upload pde files to the Arduino board but could not edit any of these files in the Arduino environment.
[I am using Windows 7] I deleted the file C:\Users\my_user_name\AppData\Roaming\Arduino\preferences.txt and everything went back to order.
I still do not know what was the issue. Any explanation?