Intermittent Message: 550 The network connection was aborted by the local system.

I'm currently getting this message during a put operation in my ftp server. Normally it works, but lately it's happening more frequently to the point I don't trust the script anymore.

Please if somebody can explain to me the possible causes other than firewall or permission issues (it works sometimes)

Attached the command file, the script file, and the log file
Thanks in advance for your help