File uploads: Warning: session_start(): Cannot send session cookie - headers already sent
I'm also getting the exact same issue on a Pantheon hosted site. This is on a more complex content type (number of fields) with the image field using the multiupload_imagefield_widget module so we can upload multiple images. The error can also appear on removing an image, where the image field is updated via AJAX. Occasionally, a WSOD will occur.

Location or Path always relates to files/styles/.... 

Tried to duplicate this on a basic content type but it was not occurring. The issue appears on the main content type 80% of the time.