'Unknown path' error when trying to access New Mailing with version 4.6.2
I resolved the issue.  Apparently there is a conflict with the Deslash option in Drupal module Global Redirect 7.x-1.5.  After unchecking the Deslash option, restarting the browser and clicking the New Mailing menu item, it works.

Please beware if you've installed module Global Redirect 7.x-1.5.  The Deslash option is enabled as a default.

Just a thought...
Although there is no need to change 'civicrm/a/' to 'civicrm/a' in source file civicrm/CRM/Mailing/Controller/Send.php, should it be changed anyway?  After looking at other CRM_Utils_System::url() calls in the module, I see that many other calls do not have '/' as the last character for parameter $path.  But it seems the parameter $path has been set to 'civicrm/a/' in civicrm/CRM/Mailing/Controller/Send.php for quite a while.