Somewhat recently (but dating to earlier releases than the most recent release of the Domain Mapping tool), we’ve noticed that logging into the WP-Admin interface consistently results in a 404 result rather than the dashboard of the site in question. (This is true regardless of whether the site is domain-mapped or not.) What’s happening is that the domain-mapping plugin is inserting the domain name of the WP-network into the path of the URL, plus a few extra subsite names for good measure, so that (for example) https://sites.globalhealth.duke.edu/dukeonehealth/wp-admin/becomes https://sites.globalhealth.duke.edu/dukeonehealth/sites.globalhealth.duke.edu/dukeonehealth/dukeonehealth/wp-admin/ once you log in via Shibboleth or via a local-user login. I’m pretty confident that this unexpected behavior is being caused by the domain mapping plugin, as it seems to be hijacking how the “redirect” parameter is getting handled after login. Note that this doesn’t appear to be an issue if you try to log into the root network site, for either Shib or local users, only for subsites.