Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%25D1%2582%25D1%2583%25D1%2580%25D0%25BD%25D0%25B8%25D1%2580%25D1%258B:%25D1%2582%25D1%2583%25D1%2580%25D0%25BD%25D0%25B8%25D1%2580%25D0%25BD%25D1%258B%25D0%25B5_%25D0%25BF%25D0%25BE%25D0%25B8%25D0%25BD%25D1%2582%25D1%258B' do
    "Hello World"
  end
end