There have been a few other reports of crashing on linux,
https://bugs.launchpad.net/calibre/+bug/1360200
but no one has got back to me with any details as yet. In any case, running without --detach is fine. --detach is only needed if you launch calibre from a terminal and want to close the terminal and keep calibre running or if the GUI environment you are launching calibre from does not provide proper stdout/stderr streams.