I’m like Cerberus FTP, and use it for long time, but it’s an old bug I looked in previous versions and in 2.31.
When user terminate connetction in time when transmission of file starts (hmm… I don’t know why, may be port 20 on client side is locked?), Cerberus FTP server waits for Idle time, then logs that connection timed out. But don’t break the connection (I’m see file in Connections and Transfers tabs, but in Statistics tab it looks as failed uploads) and locks the sendind file (it has zero length). So I can’t delete it till next system reload or terminating Cerberus FTP server by CTRL-ALT-DEL.