I have been able to replicate your problem. The VPN could be resetting the connection - which is a problem that I haven’t been able to get around with openvpn configuration options (ping). I have also experimented with FileZilla keepalives.
The timeout problem doesn’t exist with direct connections that support TLS. It is consistently 80seconds so it has to be either firewall, router or VPN. I haven’t been able to find any documentation that specifies this time out in Openvpn or VPNGate so I’m at a dead end.
So what do we do about this problem?
The FTP server and FileZilla are smart enough resume the upload after one of these connection resets. So you could just ignore the error message. You will have to be careful to make sure your upload completed successfully and that the file sizes of the uploads look right before processing.
You can investigate alternative VPNs and see if they provide any better connections? VPNGate is very slow so changing VPNs might speed up your uploads too.
You can report the issue to your ISP and see why they don’t support TLS for FTP? This maybe a big problem depending where you live and may not be possible.