Why does asset uploading fail with the error "Uploaded file was empty"?
You'll get that message whenever Craft/PHP can't upload the file (for any number of reasons).

Looks like you've covered permissions and some common php.ini environmental limitations.

If it's happening of every file regardless of file type or size, you can probably rule out any GD or Imagick related bug.

Other things to check for:


You're out of disk space on your hosting account.
You're out of allocated space for the /tmp folder on your account.
There is some firewall / .htaccess rewrite rule / Suhosin / mod_security blocking the request.


It's probably worth checking craft/storage/runtime/logs for any errors as well as your web server's error logs.