Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/info:tab:%25D0%25BA%25D1%2580%25D0%25B5%25D0%25BF%25D0%25BE%25D1%2581%25D1%2582%25D0%25B8' do
    "Hello World"
  end
end