This is possible only when a certificate coming in SAML Response does not match with the certificate configured in the plugin. Copy the certificate value shown in the Test Configuration window and paste it in X.509 Certificate field in the Service Provider tab in the plugin.
If the problem persists, please contact us at joomlasupport@xecurify.com or Click here to contact us for support. It would be helpful if you could share your use case with us.