Our product (WYSIWYG Web Builder) uses wininet to upload files to a FTP server. This has worked fine for years. Now after upgrading to IE11 many of our users have reporting FTP/wininet issues.
It's no longer possible to upload more than one file because after the first file wininet reports error 12003 and programmatically there is no way to recover from this error (trying does not work). It looks like wininet internally does not handle the response from the FTP server correctly.
Note that this problem does only occur in our software, all software using FTP through wininet no longer works. Including the standard FTP functionality built-in Windows Explorer).