I had an image field attached to a content type. Uploaded loads of images to lots of content no problems. I then added the same image field to another content type. However for some reason uploads to the second content type fail.
In Firefox I see the message "An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (64 MB) that this server supports." and Firebugs console reports "NetworkError: 403 Forbidden - http://mydomain.co.uk/file/ajax/field_sidebar_images/und/form-a0BA8npdHBpnyUpXs45pjrkPtDlu6iBefs5l6nusmv0".
In IE the Ajax throbber just throbs forever.
I know the problem isn't the image itself, or its size as the field settings are the same in both places the field is used, and I've tried uploading the same file to both content types.
I'm stumped. Any ideas ?