Hi,
As Ludof said, you may be trying to connect to a virtual web server and, or it is a very basic web server, or has some misconfiguration, because I use myself an old tiny web server that handles virtual severs without any problems.
Make sure your web server can read the "Host" HTTP header, and, in the client side, you are not blocking or spoofing it with a browser add-on.
You may need a HTTP sniffer.
I have kind of your configuration in my web server for IP Host requests and only happens when you input the IP directly in the browser. Stunnel, as a tunnel, is not the problem here. Are the client and server endpoints.
It not a DNS lookup problem.
Regards.