Jackd (via Qjackctl) worked one minute and then didn't the next.
I had some issues with some groups, and now a group is missing (though I have no idea what the group was, only its GID)
This is the message I get when I start jackd ... Not sure if it helps.

20:06:59.243 D-BUS: JACK server could not be started. Sorry
Cannot read socket fd = 18 err = Success
CheckRes error
JackSocketClientChannel read fail
Cannot open qjackctl client
Tue Sep 3 20:06:59 2013: ERROR: JackPosixProcessSync::LockedTimedWait error usec = 5000000 err = Connection timed out
Tue Sep 3 20:06:59 2013: ERROR: Driver is not running
Tue Sep 3 20:06:59 2013: ERROR: Cannot create new client
Tue Sep 3 20:06:59 2013: ERROR: Unknown request 4294967295
Tue Sep 3 20:06:59 2013: ERROR: CheckSize error size = 0 Size() = 12
Tue Sep 3 20:06:59 2013: ERROR: CheckRead error
Tue Sep 3 20:06:59 2013: Starting jack server...
Tue Sep 3 20:06:59 2013: ERROR: `default' server already active
Tue Sep 3 20:06:59 2013: ERROR: Failed to open server
20:07:06.275 Could not connect to JACK server as client. - Overall operation failed. - Server communication error. Please check the messages window for more info.
Tue Sep 3 20:07:00 2013: Saving settings to "/home/thedude/.config/jack/conf.xml" ...

Cannot read socket fd = 18 err = Success
CheckRes error

JackSocketClientChannel read fail
Cannot open qjackctl client
Tue Sep 3 20:07:06 2013: ERROR: JackPosixProcessSync::LockedTimedWait error usec = 5000000 err = Connection timed out
Tue Sep 3 20:07:06 2013: ERROR: Driver is not running
Tue Sep 3 20:07:06 2013: ERROR: Cannot create new client
Tue Sep 3 20:07:06 2013: ERROR: Unknown request 4294967295
Tue Sep 3 20:07:06 2013: ERROR: CheckSize error size = 0 Size() = 12
Tue Sep 3 20:07:06 2013: ERROR: CheckRead error