Default image is not generated
For what it's worth and consideration. Make sure your php.ini is configured correctly and that your. upload_max_filesize and max_file_uploads settings are adequate. My numbers are arbitrary, but I've had similar issues that require this general adjustment. 

Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
upload_max_filesize = 200M

; Maximum number of files that can be uploaded via a single request
max_file_uploads = 200