We don't have any extensions installed.
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.
Safari on the MAC seems to get this error regardless of the Global Redirect Fix (which doesn't appear to work).
Using 4.6.3 Removing the trailing slash lines 56, 62, 65 and 71 of civicrm/CRM/Mailing/Controller/Send.php make this work for me on safari/mac. Someone also reported that it doesn't work on some versions of IE.
I'm having the same issue for the first time but, a year later than the folks above - I'm providing this info as further clarification of the issue with Safari Browser and this redirect inside CiviMail
Our site is at Drupal 7.52/Civi 4.7.13 and another site with same issue is at Drupal 7.50 and I'm not positive which civi version
"unknown path" error when trying to access New Mailing Only seems to occur with Safari on a mac (and on an iPhone)
Further clues: The url box at the top of the browser very quickly cycles through the following three urls when I choose "New Mailing"
On other browsers, the url's cycle through just the first two of these, and don't reach the third which is the thing that causes the "unknown path" error. This is happening on at least a couple of sites as I note above, and doesn't seem to be directly associated with Drupal or CiviCRM updates.
Perhaps there has been an update to Safari that's reawakening an error from a year ago? I'm on Safari Version 10.0.1 (11602.2.14.0.7)but I don't know how to tell when it was updated..
Per above, I tried changing 'civicrm/a/' to 'civicrm/a' in lines 53,59,62,68 of my ...sites/all/modules/civicrm/CRM/Mailing/Controller/Send.php file and it didn't change the error.