Marco,
Surely it was a environment problem. When I connect with a different user (at Operational System), the problem doesn't occur. Using this workaround we solved our problem.
Thanks a lot.
Dear friends,
I have a problem when try to connect to oracle using PL/SQL Developer 6.0.5. It shows me a little window, without message, containning just a button "Ok", after I inform user, passord and database at logon dialog box.