This issue is very frequent and recurring. More with "Manage Add-ons" and less with "Find Add-ons".
Always redirected to this page and never reach the page in demand [manage app] - /authenticate.action?destination=%2Fplugins%2Fservlet%2Fupm%3Fsource%3Dside_nav_manage_addons
Sometimes, get stuck with new app install using finding app option. See the screenshot.
This is affecting us finding and managing new plugins or updating them. Any help here!