I suspect this is because it’s an old version of snowflake-proxy. Looks like it’s 2.5.1, it has not been updated for a year. Check out this post and this issue.
Can you check what version it is for you?
No, the error that you’re seeing is not related to restricted / unrestricted NAT. The connection to broker is a regular HTTPS connection and it should not fail for any NAT.
I also tried to install snowflake on Debian myself, and got an even older version. There were breaking changes since then, so it doesn’t work, yeah.