Symptoms
With SSO enabled, a user navigates to a CMS page and is presented with a modal login window and there will be a "401" error in the network tab of the developer tools for a GET transaction to the "guided_tours/autolaunch" API
Release
London
Cause
There is a Guided Tours record that does not have a value for the mandatory field "Context". This field was not mandatory in previous releases, this could be why there are blank values for this field in instances that upgraded to London.
Resolution
You can either delete the Guided tour record, or provide a value for the "Context" field