There are many reasons for this error, you can try the following steps to solve your problem:
- Please check whether all the clients have this error. If so, then it may be that the client does not have permission to access the FTP server. Please check the permissions on the FTP server.
- Turn off the firewall on the client before testing.
- Go to internet options>Advanced tab> uncheck Use Passive FTP(for firewall and DSL modem compatability).
You can also refer to this link about FTPS requires special firewall configuration: Windows Firewall setup for Microsoft FTP Publishing Service for IIS.
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