Hi Aleksei,
Please setup the FTP server's firewall support for PASSIVE MODE.
Two firewall rules are necessary for passive FTP to function properly:
The firewall must allow connections on port 21.
The firewall must allow connections to the ephemeral ports used by the FTP application.
Please refer to article below.
https://documentation.meraki.com/MX/NAT_and_Port_Forwarding/Active_and_Passive_FTP_Overview_and_Configuration
If there is still the issue, please report the PB issues via our standard support ticketing system to ensure it is being properly received and tracked by our tech support:
https://www.appeon.com/standardsupport/.
And let's schedule a remote session to debug it. We would recommend you to make the remote session during our working time which is 9:00 ~ 17:00 (we are in UTC+8 ) thus we can get the immediate help from the other team if necessary.
Regasrds,
ZhaoKai