Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/classes:%D0%BF%D0%B2%D0%BF_%D0%B8_%D0%BF%D0%B2%D0%BC_-_%D0%B1%D0%B0%D0%BB%D0%B0%D0%BD%D1%81' do
    "Hello World"
  end
end