I try to understand all the steps you are following to replicate the error Are these the steps you do?

• Start a Jenkins instance and install the SAML plugin
• Configure the SAML plugin manually
• Check that the configuration is valid and the Authentication works
• Go to JCasC on the manage Jenkins and export the configuration (<--I guess your error is here)
• Stop Jenkins
• Copy the JCasC file to the Jenkins Home folder
• Start Jenkins

In which of these steps do you see the error?

timja Any idea what this could be?

Attribute is not a describe-able https://github.com/jenkinsci/saml-plugin/blob/master/src/main/java/org/jenkinsci/plugins/saml/user/SamlCustomProperty.java#L45

ifernandezcalvo, the error occurs after I reload the JCasC config by restarting Jenkins. After the server comes back up SAML is no longer configured in the Configure Global Security screen, and when I view the JCasC config I see the "No configurator found" error message.

I am trying to replicate it locally but I cannot on Jenkins core 2.277 and the SAML plugin from sources, I will try with Jenkins core 2.303.2, what it is weir for me it is that fails on the class "org.jenkinsci.plugins.saml.user.SamlCustomProperty", this class is used to save a custom property in the user, it is not used on the configuration classes so I dunno why it is used in the JCasC load it should not.

jenkins:
  securityRealm:
    saml:
      binding: "urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect"
      displayNameAttributeName: "displayName"
      emailAttributeName: "mail"
      encryptionData:
        forceSignRedirectBindingAuthnRequest: false
        wantsAssertionsSigned: false
      groupsAttributeName: "groups"
      idpMetadataConfiguration:
        period: 0
        url: "http://127.0.0.1:58080/simplesaml/saml2/idp/metadata.php"
      maximumAuthenticationLifetime: 86400
      usernameAttributeName: "uid"
      usernameCaseConversion: "none"

I cannot replicate it, in my test environment the JCasC works as expected https://github.com/kuisathaverat/jenkins-issues/tree/master/JENKINS-66828 due to the class that fails in your environment is a User property that does not have anything to do with the SAML configuration classes I'd recomment to remove all users folders at JENKISN_HOME/users/* and try again.