I am logged in to Portal for ArcGIS on the machine that hosts it using its internal address (e.g., https://<internal.dns.name>:7443/arcgis/home).
When I click on Organization, then click on Settings, I get a popup windows that asks me to sign in to the Web Adaptor address (e.g., https://<external.dns.name>/portal). An ArcGIS Enterprise login popup window appears, into which I enter my credentials and click Sign In. This window then goes blank, but the original window in which I accessed Portal remains open, and I'm then able to navigate the Organization Settings.
In the browser's developer tools, I see that a call is being made to /sharing/rest/oauth2/validateRedirectUri, and the redirect Uri being passed is the internal address.
In the Portal Administrator Directory, under Security/OAuth/GetAppInfo, I can see that the internal address is listed in the redirect Uris (twice!), but without the internal port qualifier (:7443). I updated one of the redirect URIs to include the port number, but the issue persists.