Just noticed a couple of spelling errors in one of the language files (admin side) and also a missing definition. Details can be found below.

admin/includes/languages/english/subscription_manager.php

Lines 30-31 should be changed to:

Code:
define('TEXT_INFO_HEADING_NEW_SUBSCRIPTION' , 'New Subscription');
define('TEXT_INFO_HEADING_EDIT_SUBSCRIPTION' , 'Edit Subscription');
Also, TEXT_IMPORT does not exist. So, the following code needs to be added at the end (before the closing ?>) of this file.

Code:
define('TEXT_IMPORT', 'Import');
Hope this helps!