Similar issue i had encounter recently. I had followed the below steps to resolve the problem, looks like tsn error is because of many reasons. you try this and let us see.
Instead of updating the tnsnames.ora manually, I suggest to create a connection string through Net Manager.
1. Open Net Manager.
2. go to Local > Service Naming.
3. After selecting Service Naming, click on Edit menu > Create
4. Give a Net Service Name (any user defined name)
5. Select TCP/IP (Internet Protocol)
6. give the Host Name (IP Address or Host machine name)
7. Service Name : (it should be the global_name of the database)
select * from global_name
8. After completing it, go to File and save Network configuration.
9. Select the connection string created and clikck on command menu > test service.
10. Give the valid user name and password for connecting.
While connecting DB thru SQLPLUS, give the username/password@connectionstring
(conn string created in step 4)