Finally, eight weeks after my initial request for compensation, I received an official denial by snail mail (the letter was dated April 13, but I received it on May 4). Here is the reason they gave:
"Please let me explain why cash compensation is not applicable under European Regulation EC261. Because all routine preventative maintenance actions were performed on the aircraft as scheduled, and the extended delay could not have been reasonably predicted or avoided, the event is considered force majeure and exclusionary to the mandatory compensation rules."
In other words, it's God's fault that they had a faulty aircraft. Perhaps I should apply to the Vatican for compensation!
With this kind of reasoning, it is difficult to imagine a probable scenario in which an airline would ever be required to provide compensation. Let's see if ENAC agrees that this falls under force majeure.