Judging from your description, it sounds like you're using the web-ftp. I'm under the impression that the blank page you're getting is a 510 (gateway timeout), as Miles seems to be implying, simply because it's taking so long to load the next page - unfortunately, I can't confirm that. (If it takes longer than a few minutes to load a new page, some gateways/proxies will disconnect you from the webpage, and usually return an error message containing the above code.)
As a solution, I'd recommend getting an FTP client, and preferably one that allows you to append files, or resume those uploads (most clients can do this). Not only will you be notified of a disconnection sooner, but it will enable you to continue where you left off.
