Did you attach a large file? If so, then sometimes you get this error when the file is too large. Storing the file on a file sharing service (dropbox, mediafire, etc...) and using the share link in the ticket submission will then work.
Hopes this will work for you.
Edit: PS, don't use one-shot-links! The support system of SD is weird (in this context) it will hit the link multiple times. So an one-shot-link will invalidate on the first passage, and prevents the support guys/gals at SD to actually download the file.
