Some users of Windows 8.1 have found that following an update to Internet Explorer 11, their FTP clients have begun timing out midway through transferring files.
This is a known issue, is caused by an alteration that Internet Explorer 11 makes to the framework that FTP clients such as FileZilla rely upon.
Microsoft have acknowledged the issue, and have said that they are working on a fix.
In the meantime, the issue can be temporarily resolved by removing the Internet Explorer 11 update until Microsoft issue a patch for the problem.