If you can download files from a remote server but not upload to, the most common reason is that the server has run out of disk space, or you’ve exceeded a storage quota assigned to the FTP user or group (for example, your company). If you think this might be the problem, try deleting some files from the remote server and try again to upload files.
Depending on the server, you might be able to check the disk space currently available to you in a user dashboard. If not, check with your web hosting service or server administrator.
Very occasionally, a server’s file system might experience technical difficulties to the point that all files are read-only – you can look at them and download them, but no changes can be made to existing files or new files added. Contact your server administrator or web hosting provider for assistance.