I don't think this is anything to do with Webspeed. How large is the CSV file? Could it be a timeout issue on the server side? Unfortunately I don't know Tomcat well enough to be able tell you what setting you would need to tweak to increase it.
Also, do you use the default browser file upload button or some custom JS to upload the file? I've had a lot of success with this code which seems to work well across browsers: http: //code.google.com/p/upload-at-click/
Lee