I am using opera 11.52 right now. For quick local web servers, I used MoWeS Portable. I set apache to listen on port 72, so in opera I typed
and I am not having any such error.
However, when I changed the apache configuration to listen on port 70, I can replicate your error.
It's possible that because HTTPS is typically used for HTTP over SSL (HTTPS) that trying to run a url as a regular http://... gives an error. Though, in my testing I can run a non HTTPS server connection on 443.
Now, port 70 in particular is for the (virtually) deprecated gopher protocol which was more common before the rise of WWW http://en.wikipedia.org/wiki/Gopher_(protocol). So maybe they now block port 70 in particular.
As for your 443 error posted on the opera boards, perhaps as the first responder noted you were trying to connect to a port expected SSL and your server is not setup to automagically handle that for you. Did you get the same unsafe port error?
In your case it could be a combination of things, perhaps you can provide some more information to clarify.