<update>
I re-read all of our emails and I see that I suggested you update to the latest build at the time which resolved the issue. That build happened to be v4.2 build 1878.
I suspect that this might be related to the changes we made with the default TCP/IP keep alives but I am not 100% positive. You can read up on this setting and how to change it
here.
If you want to download the previous version that worked you can download the portable edition of that build using this link below.
FlashFXP42_1878_Portable.exe
update 2
I just noticed you're uploading a video file, if the FTP server performs additional processing after upload you may need to disable the "Preserve file date/time" setting and more information can be found
here.