Another possibility is that your PHP settings are set too low to allow the ZIP file to upload. Specifically, PHP may be configured with a maximum file upload size limit at 8 megabytes or less. The plugin ZIP is about 13.2 megabytes. Because of this, the web server is rejecting the upload and WordPress is giving you an ambiguous message in response. To fix this, you can either increase the limit or install Intense using FTP.