I was able to sign up one user via SSO, but the succeeding ones just fail. Netmaker is showing this message:

This is a known issue that will be fixed in the future. The issue triggers when you press the back button on your browser after signing up, or when you press the back button after an unauthorized attempt to signin with a non-admin account

For now if you were able to signup a user via SSO, rest assured that your OAuth is properly configured despite Netmaker telling you otherwise.
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As a workaround, just navigate back to the login page and then refresh the page or do a hard reload by pressing Ctrl + F5.