553 'destinationflatfile.txt;application:x–wmflatfile': cannot write.

I have set up an FTP port in IS with port number 8111. While trying to put a file using pub.client:ftp, I am facing the following error.
553 ‘destinationflatfile.txt;application:x–wmflatfile’: cannot write.
I am using the administrator access to the FTP service.
I am able to List files and get files already present properly without any problem.
One thing noticed is IntegrationServer\userFtpRoot gets Read-Only locked automatically. Is this done by IS and how to put a file into the Folder.

Please Advise,

Based on the error, Is your FTP user has PUT access (write to allow in that particular folder permissions) and so this could be Q to your sys admin question:

Please check with them and revert back.