This following error shows when accessing any of the CopyMove pages.
CAUSE
An unknown error can of course be anything. But this is also just the general error message that SharePoint shows in a production environment in order to not compromise any system information to end-users.
RESOLUTION
This error usually occurs because of a missing resource file - see the section "The resource object was not found" for more information. To see the exact error message, open the web.config file of the SharePoint Web application and set the CallStack attribute to true on the SafeMode element, and the mode attribute to Off on the customErrors element. Then repeat your actions to reproduce the error.