Username conflicts
GitHub Enterprise Server normalizes the SCIM userName attribute value that is sent by an identity provider (IdP) in a SCIM API call to create each person's username on GitHub. If multiple accounts are normalized into the same GitHub username, a username conflict occurs, and only the first user account is created. For more information, see Username considerations for external authentication.
In an enterprise with managed users where SCIM is generally functioning properly, individual user SCIM provisioning attempts sometimes fail. Users will be unable to sign in until their account is provisioned to GitHub. These individual SCIM user provisioning failures result in an HTTP 400 status code and are typically caused by issues with username normalization or username conflicts, where another user with the same normalized username already exists in the enterprise. See Username considerations for external authentication.
SAML authentication errors
If users are experiencing errors when attempting to authenticate with SAML, see Troubleshooting SAML authentication.