Oracle documentation states the following cause, along with what it suggests to be the sole solution:
ORA-06401 NETCMN: invalid driver designator
Cause: The login (connect) string contains an invalid driver designator.
Action: Correct the string and re-submit.
This usually means that the command syntax is incorrect. However, other causes exist:
1. improper authorization for the user issuing the command
2. invalid connection string for the target database version
3. ^M characters present in the file tnsnanes.ora file. These can be seen by viewing the ascii file after it has been converted to binary or by editing the file in a UNIX/Linux editor, such as vi. These characters are usually created by editing the file with a non-Linux/Unix editor and then transferring it by FTP to the server.
Use the following vi editor command to remove all of these characters from the file: