Our WebFTP service is still in beta testing, and is recommended only for emergency FTP access as it is still in development. As WebFTP is also run within your browser window, errors may occur as it is browser-based, and not a standalone program. The error you're seeing is common; I've run into that same error myself as well.
Since WebFTP is still a beta product, and it is currently giving you that error, I would recommend using a standalone, stable FTP client to upload your files. I use the FileZilla client with no problems, but you can find a list of recommended clients in our wiki, at:
Please try uploading using an FTP client, and if you run into any other errors, please email our team directly with instructions to replicate the error. Thanks!