Access Denied error uploading using FTP with additional users
Symptoms
When you try to upload files using FTP with additional users that were created in the User Manager Section of the Control Panel, the system can and may respond with an Access Denied error.
Cause
The IIS FTP server does not have the ability to map additional users to your webroot.
Resolution
When you use any other user other than the main user account to FTP, you must change your directory to /[account_username].
Content retrieved from: https://support.appliedi.net/kb/a409/i-have-problem-uploading-using-ftp-with-additional-users-access-denied-error.aspx.