That's a too broad issue to answer without knowing anything about their network.
The connection can be blocked by their firewall for example.
I've asked them to try a different client. They get the same error with FileZilla, so not a WInSCP problem. DO you have any ideas what could cause this anyway?
Can the client connect with any other FTP client?
What makes you believe this is a problem with WinSCP?
We have our servers behind a firewall. FTP is configured for TLS/SSL connection only.
The client can connect to server named HOST01 and upload files but gets the above error when trying to connect and upload to server HOST02. Bothe servers have FTP and permissions configured in the same way.
We have other clients that can connect and upload to HOST02 without any issues. I can also connect and upload to HOST02 OK.
WinSCP is 5.9.3 build 7136 connecting to a Windows 2012 server via Windows 7 or Windows 10.
Settings are FTP with TLS/SSL Explicit Encryption.
Log files attached.