Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%25D0%25BF%25D1%2580%25D0%25B0%25D0%25B2%25D0%25B8%25D0%25BB%25D0%25B0_telegram:start' do
    "Hello World"
  end
end