ORA-12638: Credential retrieval failed

Toad threw this error today, but I was able to connect to the database using SqlPlus.

The problem is coming from your sqlnet.ora

Check if your SQLNET.AUTHENTICATION_SERVICES.  It should be set to NONE instead of NTS

Old entry
SQLNET.AUTHENTICATION_SERVICES= (NTS)

Correct Entry
SQLNET.AUTHENTICATION_SERVICES= (NONE)