And this should be enough proof, I have used it before. By the way, you do not need to use an agency. Write to Lufthansa in their standard contact form that you are requesting compensation according to EU261 for your flight (mention date and flight numbers), and that since the delay was caused by mechanical reasons they cannot claim it was an extraordinary circumstance. Tell them that if they don't send you a check within 21 days you'll pursue it in court (be vague, it's an empty threat anyway). They will most likely reply that it was extraordinary, stand your ground and repeat the deadline. They'll send you a check.