Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%D1%82%D1%83%D1%80%D0%BD%D0%B8%D1%80%D1%8B:%D1%82%D1%83%D1%80%D0%BD%D0%B8%D1%80%D0%BD%D1%8B%D0%B5_%D0%BF%D0%BE%D0%B8%D0%BD%D1%82%D1%8B' do
    "Hello World"
  end
end