Okay, here's the backtrace. Hopefully this'll be useful in some way...
(gdb) bt
#0 0x0000000000000001 in ?? ()
#1 0x00007f0b06e933a3 in QObject::disconnect(QObject const*, char const*, QObject const*, char const*) ()
from /opt/calibre/libQtCore.so.4
#2 0x00007f0b05fd4031 in ?? () from /opt/calibre/libQtGui.so.4
#3 0x00007f0b05fd5263 in QToolButton::mousePressEvent(QMouseEvent*) () from /opt/calibre/libQtGui.so.4
#4 0x00007f0b05ba8709 in QWidget::event(QEvent*) () from /opt/calibre/libQtGui.so.4
#5 0x00007f0b05b523ec in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
from /opt/calibre/libQtGui.so.4
#6 0x00007f0b05b591ab in QApplication::notify(QObject*, QEvent*) () from /opt/calibre/libQtGui.so.4
#7 0x00007f0b02697de6 in ?? () from /opt/calibre/PyQt4.QtGui.so
#8 0x00007f0b06e7ce7c in QCoreApplication::notifyInternal(QObject*, QEvent*) ()
from /opt/calibre/libQtCore.so.4
#9 0x00007f0b05b58386 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) () from /opt/calibre/libQtGui.so.4
#10 0x00007f0b05bd807b in ?? () from /opt/calibre/libQtGui.so.4
#11 0x00007f0b05bd6d8a in QApplication::x11ProcessEvent(_XEvent*) () from /opt/calibre/libQtGui.so.4
#12 0x00007f0b05c00cc2 in ?? () from /opt/calibre/libQtGui.so.4
#13 0x00007f0b066678c2 in g_main_dispatch (context=0x3f8ee00) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:1960
#14 IA__g_main_context_dispatch (context=0x3f8ee00) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2513
#15 0x00007f0b0666b748 in g_main_context_iterate (context=0x3f8ee00, block=<value optimized out>,
dispatch=<value optimized out>, self=<value optimized out>)
at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2591
#16 0x00007f0b0666b8fc in IA__g_main_context_iteration (context=0x3f8ee00, may_block=1)
at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2654
#17 0x00007f0b06ea6423 in QEventDispatcherGlib:
rocessEvents(QFlags<QEventL oop::ProcessEventsFlag>) ()
from /opt/calibre/libQtCore.so.4
#18 0x00007f0b05c008be in ?? () from /opt/calibre/libQtGui.so.4
#19 0x00007f0b06e7b7a2 in QEventLoop:
rocessEvents(QFlags<QEventLoop::Proce ssEventsFlag>) ()
from /opt/calibre/libQtCore.so.4
#20 0x00007f0b06e7bb7c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsF lag>) ()
from /opt/calibre/libQtCore.so.4
#21 0x00007f0b06e8080b in QCoreApplication::exec() () from /opt/calibre/libQtCore.so.4
#22 0x00007f0b02698b4b in ?? () from /opt/calibre/PyQt4.QtGui.so
#23 0x00007f0b0b1166b7 in PyEval_EvalFrameEx () from /opt/calibre/libpython2.6.so.1.0
#24 0x00007f0b0b11700b in PyEval_EvalFrameEx () from /opt/calibre/libpython2.6.so.1.0
#25 0x00007f0b0b1184e5 in PyEval_EvalCodeEx () from /opt/calibre/libpython2.6.so.1.0
#26 0x00007f0b0b1167db in PyEval_EvalFrameEx () from /opt/calibre/libpython2.6.so.1.0
#27 0x00007f0b0b1184e5 in PyEval_EvalCodeEx () from /opt/calibre/libpython2.6.so.1.0
#28 0x00007f0b0b1185b2 in PyEval_EvalCode () from /opt/calibre/libpython2.6.so.1.0
#29 0x0000000000401c56 in _start ()