I got his error message on a 'move' task :
10/17/2017 10:13:05.339:tasks.CopyTask.Move LOGFILE:OUT:FTPClient:CrushTask7java.io.BufferedReader@37eecc2d:500 'MFMT 20171017101301 [...].log': command not understood.
How this issue can be explained ?
You interacting with a bad FTP server...one that doesn't implement standard commands that have existed for probably 15+ years now.
Your Copy step has preserve modified time enabled...turn that off as the server your talking to doesn't support it.