@Loong Which is itself marked as a duplicate already... More duplicates are here http://german.stackexchange.com/questions/30616/past-participle-of-modal-verbs and here http://german.stackexchange.com/questions/70/why-no-perfect-participle-sie-hat-sich-scheiden-lassen - Some of the answers refer to the Ersatzinfinitiv (which is the core of the answer), some don't.
– tofroJul 02 '16 at 11:07