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An error has occurred when trying to access IdpInitiatedSignOn.aspx to test SAML authentication on AD FS 4.0 (Windows Server 2016) | A Windows System Admin's Blog
So usually one of the first things I do after initially setting up an AD FS environment (among others) is to test the Metadata (navigate to https://your.adfs.server/ federationmetadata/2007-06/