I would try to use either the free and open source CURL.EXE a command line utility that can do FTP and HTTP operations. It is extremely flexible and robust. You can start CURL from a QlikView Load Script by using the EXECUTE statement. You will have to allow the load script to run external programs.
You could also use regular FTP.exe in Windows - a command line utility that comes with Windows. And finally you could also use PowerShell - if you have the execution rights set correctly to execute PowerShell-scripts. The script is just a line or a few lines long.
I agree - but I guess that the FTP-support in QlikView hasn't been maintained much the last few years and it does not have a high priority from Qlik's side at the moment. FTP isn't always entirely straightforward so that is why sticking with some well-proven popular pure FTP-products is necessary if you get into problems with Qlik's simple approach.
So as I said: CURL is an option but I forgot to mention something else that might be even better because it has both a Windows UI and can be run from the command-line: