After Summer '15, Manage External User > Login as User takes you to a custom Login Page, an inactive force.com Site page, or may show an error
|Knowledge Article Number||000220790|
|Description||After Summer '15, you may find the "Log in to Community as User" button may take you to a different page (most likely, a login page:
|Resolution||In Summer ' 15, if the "Active Site Home Page" was updated to be the login page then this setting in Community Management will reflect the same page.
Ensure you update your pages and references accordingly.
Force.com Sites Settings:
Community Management Settings: