Why are users redirected to the logout URL when authenticating via SSO with SAML?
The common reason for this behavior is the update or change of your server's SSL Certificate, the SSL installed on your SSO server side.
Because the certificate has a new fingerprint, you need to update the existing one from your Support account.
If you don't have access to the data of the new certificate, decode the new SSL using an online tool, such as CSR Decoder And Certificate Decoder. Then, grab the new SHA2 fingerprint that looks similar to this:
In Admin Center, navigate to Account > Security > Single sign-on.
Update the Certificate fingerprint field and Save.