Why when using the same FTP username and password to login into FTP server, I can upload files and my client can't?

Question

I created FTP server using vsftpd package on Ubuntu server on AWS EC2. I gave the user local_umask=022. I login using the FTP username password, I'm able to upload and delete files. I have tried using FileZilla for Windows and for Mac. Meanwhile, my client is using the same username password on FileZilla on Windows, they can login, they can list the files, but they cannot upload to the same folder, with error message "Could not start transfer". Why is this happening?