Would you be willing to let us look at the .ZIP file that the plugin is creating for you?
The plugin creates the ZIP archive based on the ZipArchive class that is *usually* compiled into PHP by most web hosts that support PHP.
As you can imagine, there are a variety of things that could affect a successful ZIP file being created.
If the script timeout setting is to low, the ZIP archive may not be finishing and not completing a valid archive.
One thing that you could try is to increase the script_timeout directive in PHP, if your web host allows you to do that.
In version 4.0 due out sometime this year, we'll include an option to adjust script_timeout via ini_set() (still not a guarantee that web hosts' will support ini_set).
MyCodeTree Support Team