This error message occurs because the user record attempting to launch Composer is frozen.
Cause
This Invalid Grant: authentication failure error messages appear in the Composer UI when a user launches their Composer button. The error occurs if a Salesforce user is frozen and attempts to launch Composer.
What to Do
Unfreeze the Salesforce user to resolve the error message. For more information on freezing or unfreezing User Accounts, seeFreeze or Unfreeze User Accounts.