Failed transfers when using more than 2 simultaneous transfers

[Beosar]

When setting "Maximum simultaneous transfers" to more than 2, some files always fail to upload on the first try. Sometimes it asks to overwrite an "existing" file, which has an unknown size.

Download these files (I cannot attach them because of size), unzip them, set "Maximum simultaneous transfers" to 10, and upload them all at once to a server to reproduce the error: ​https://download.beosar.com/public/FileZilla_Upload_Bug_Files.zip

Transfer mode is "Binary". The files that failed are all 7 .lua files in the folder "plugins/server/cubeuniverse".

This bug occurred on 2 servers on the Internet, while it did work on my local Raspberry Pi.

A fast upload speed (tested with 40 mbps) might be needed to reproduce the error.