I was trying to post this answer to stackoverflow but I was banned to do that. I believe the answer below may be helpful so I publish it at least this way:
I was running sinatra via
Rack::Handler::WEBrick.run MyServer, webrick_options
and after installing sinatra-contrib gem suddenly I was unable to to access my app from outside (localhost was working perfectly). All the hints with
-o were not working for me. What finally helped was to put inside
webrick_options hash following key and value:
:Host => 'myserver.com'