Troubleshooting Sign-In using Twitter
From WHMCS Documentation
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You may receive the above error message when attempting to configure your Twitter API Credentials. It indicates that the values provided are not able to successfully authenticate with the Twitter API. This indicates that either the credentials are invalid or 'Callback Locking' is enabled inside the Twitter app. Checking the credentials, correct them if required and ensuring that 'Callback Locking' is not enabled should resolve this.