"No longer starts" - what an euphemism. It crashes really bad.
Well, given that it's from debian/unstable - maybe Adrian who packaged
it (and is also subscribed to this list) has a hint. Otherwise open a
terminal and type `reportbug jackd2`. "unstable" is - well -
occasionally "unstable" :)
A common cause for jackd segfaulting is that you one has some [self
compiled] 2nd version of jack around on the system - but the trace does
not support that..