SAML access was not possible. In the log I can find the message: The status code of the Response was not Success, was urn:oasis:names:tc:SAML:2.0:status:Responder
This error says that the IDP refused access for this user. Please check at the IDP (e.g. ADFS) if only certain user are allowed to access Signavio Trust Party.
KBA , BPI-SIG-CA-SEC-SAM , SAML 2.0 for SAP Signavio , How To