Thanks Harry, you are amazing !!! you saved my day.
I would need one more help => for my knowledge, when do we use this "(SERVER = SHARED)" in tnsnames? is this for Shared server config instead of dedicate connection mode?
I presumed my DB was in shared server config as the spfile parameter shared_servers=5
the shared_server parameter allows shared server connections. But the database can always receive dedicated server connections.
From the database's side, 'shared server' vs. 'dedicated server' is NOT an either/or situation -- they are NOT mutually exclusive.
Now that we know you had configured for shared servers, perhaps you could try increasing the value of shared_servers and set your tnsnames back to use a shared server connection.
And do some more reading about the pros and cons of shared server and the reasons you might choose to use - or not use - a shared server connection. A good place to start would be the official docs at tahiti.oracle.com