Strange; I suspect it may be to do with your installation URL. You are using /admin, rather than the default lists/admin, which means that $pageroot may be incorrect, or other settings relating to the URL of the installation. See also [SOLVED] Pageroot problem after upgrading to 3.3.7.
You could use the web browser debugger to see what is happening with the redirect, eg if it’s due to JS or the webserver.