Solved this on my installation!
I was also getting this error in a v1.4 installation. Any zip download
would be 276 bytes and empty. The problem in my case was the document
root specified in apache. My RS installation is in my home folder, and
the zip files it creates are in the root tmp folder. I didn't get this
error on a test install in the var/www directory, so I changed the
server doc root to var/www and created a symbolic link to the RS
installation. Nothing is changed from the frontend, all addresses and
links are the same, and now the zip d/l is working.