This is a work related techie blog post for the benefit of others who may experience the same problem.
We have a production Salesforce Org and a number of sandbox Orgs, all set up with a Salesforce “My Domain” and configured to use Single Sign On authenticating against Microsoft ADFS. On the weekend, after our sandbox Orgs got upgraded to the Summer ’19 release, we were unable to login to any of our sandboxes using Single Sign On. We were just getting a very unhelpful “An error occurred” message on the sign-in screen.
Comparing the Single Sign On settings in our Production Org (which was still working), I noticed that where the “Login URL” SAML endpoint used to have an “so=OrgID” parameter, this was now gone in the updated sandboxes. Jumping on to our ADFS management console, and editing the relevant Relying Party Trust to remove the “so=OrgID” parameter from the Endpoint was all that was needed to fix the problem.
Curiously, when I checked the release notes for the Summer ’19 update (which is 480 pages long!), there appears to be no mention of this change in Single Sign In configuration.