Please navigate to the window below to check whether FTP is running or not. Please also check whether there is antivirus or firewall block the default FTP port 21 in your environment.
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If you already enabled it and are still not able to connect it, please try to use FileZilla to confirm the cause.
If you saw the error message like below:
It means that the maximum number of client is reached. It is suggested to close PC/laptop where the FTP service is not executed or modify the max clients on web UI then reconnect again.
If you saw other error message like below:
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It means that the maximum number of connection for host is reached. It is suggested to close current connection then reconnect or modify the max clients per host then reconnect again.