There is a similar question here which would likely help you answer your question: https://stackoverflow.com/questions/42168773/how-to-resolve-preflight-is-invalid-redirect-or-redirect-is-not-allowed-for
You want to ensure that when your browser is making a cors pre-flight request that the response comes back with a 200. If the server tries to redirect for any reason when the preflight request is sent, then you will get an error, which is enforced by your browser.
Your error message says: 'https://saml-provided.not.real' (redirected from 'https://my-site.not.real')
I would recommend using CURL to construct a preflight request to see how the server responds when you make the request. If you get back a 3xx status code, follow that thread.
However, this is typically only an issue when attempting to use fetch api, or the older XMLHttpRequest api from within a web app. Since your error message says that SAML is somehow involved I would also look in to why you are trying to do a saml authentication using something other than plain old HTTP posts from <form> elements which do not do preflight requests.