So ... that http error is basically WP saying "Aaaaaugh!" It's my second least favorite error out there right now.
- Image cannot be parsed by our image compression/resixer (imagemagick normally, sometimes GD), so it can't generate thumbnails
- No room in /tmp/ for your files to be processed
- Plugin/theme conflicts
- Something else
If you're having it with LARGE files exclusively, then it's probably the tmp folder. You can put in a ticket and ask us to check how full it is, but if it's not very full and you're on Shared, you're out of luck If you're on VPS/Dedi there are things we can do to mitigate it, but in general, it's an issue with the tmp folder.
If you're only having this when you try to upload a lot of moderately sized images, upload them in smaller chunks. Same reason