If you're using a host name for the FTP, also try the IP.
I had issues with a host one time, and it turned out it was IPv6 routing issues on their end. Using an IPv4 worked.
If you have a firewall, try checking its logs / disabling it, but I guess you already tried that.
You could also try 'HTTPing' to test for connectivity issues. I read that regular ICMP ping isn't really meaningful in a lot of cases. It's using a different protocol and its results can be misleading. (I think httping needs a http web server on the IP, but I'm guessing yours has one.)
https://www.vanheusden.com/httping/