If you are getting this error, check your PHP.ini locations by using the script Cody from Blesta has created.
You can get it from: https://licensecart.com/public/include_path_test.zip
Expected output:
php test_include_path.php
Current include path: .:/usr/bin/pear
Modifying include path to include: /var/www/html/include_path_test/includes
Include path is now: .:/usr/bin/pear:/var/www/html/include_path_test/includes
Success. Attempting to a include file. Should print "SUCCESS!!" below...
SUCCESS!!
If you don't get a Success your PHP.ini isn't set-up correctly.