After upgraded to version 6.1 the tnsnames.ora could not been read.
Starting the new interface it said, it could not find the description of the database and the drop down list in the connection window is empty. Trying to create a new connection had no success.
Suggesting an invisible character in the tnsnames.ora that causes this error.
1. Rename the existing tnsnames.ora file.
2. Create a new tnsnames.ora file manually.