I would like to clarify this a little, in order to be able to give a more useful response.
You say “<b><i>after doing the connection</i></b>, when I try to log into the application …”. Does that mean that you can successfully connect to the database, but you cannot log into your application ?
If yes, how is the application’s user/password information stored in the database ? is it encrypted ?
If you cannot connect to the database from forms, have you tried to connect from SQL PLUS with the same user/pwd ? How did you upgrade the database ?