Ah Wait. I opened the zip file in a text editor and it gives me this message:
<b>Fatal error</b>: Allowed memory size of 268435456 bytes exhausted (tried to allocate 129259132 bytes) in <b>/home/public_html/wp-content/plugins/backwpup/libs/dropbox.php</b> on line <b>259</b>
Guess I need to figure out how to up the memory available, but I wonder why the error is happening in dropbox.php as downloading the zip shouldn't need dropbox to be involved.