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,
Suresh

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.

HTH,
RMG