Paypal ipn issues after upgrade to 4.6.15.
Why is IPN failing on Pantheon?

Check out CRM-17212: IPN and other scripts break on Pantheon due to session handler although it does look like you have a similar patch in effect. Compare your changes with those merged into the 4.7 branch, or consider trying 4.7.

How do I debug on Pantheon?

On Pantheon hosting, you can inspect your application's error logs in:

logs/php-error.log


For additional tips on debugging PHP errors, see also:


Where should one look for logs when debugging a new problem? and 
How do I debug CiviCRM when I hit a blank page / white screen / fatal error? 


Pantheon specific docs on logfiles are:


https://pantheon.io/docs/debug-log-files/
https://pantheon.io/docs/php-errors/