Additional information: When the message box appeared, I used gdb to attach to the kmail process to get this backtrace:

#0  0xb76e7424 in __kernel_vsyscall ()
#1  0xb5eb6bac in poll () from /lib/libc.so.6
#2  0xb45bef90 in g_poll () from /usr/lib/libglib-2.0.so.0
#3  0xb45b2b16 in ?? () from /usr/lib/libglib-2.0.so.0
#4  0xb45b2dfd in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#5  0xb61f0ba6 in QEventDispatcherGlib::processEvents (this=0x9052ae8, flags=...) at kernel/qeventdispatcher_glib.cpp:422
#6  0xb6500a2c in QGuiEventDispatcherGlib::processEvents (this=0x9052ae8, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#7  0xb61bf5ff in QEventLoop::processEvents (this=0xbfbc6a80, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:149
#8  0xb61bfa25 in QEventLoop::exec (this=0xbfbc6a80, flags=...) at kernel/qeventloop.cpp:201
#9  0xb69ca7dc in QDialog::exec (this=0x920c240) at dialogs/qdialog.cpp:552
#10 0xb743364c in KMessageBox::createKMessageBox (dialog=0x920c240, icon=..., text=..., strlist=..., ask=..., checkboxReturn=0x0, options=..., details=..., 
    notifyType=QMessageBox::Critical) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdeui/dialogs/kmessagebox.cpp:337
#11 0xb7433f2b in KMessageBox::createKMessageBox (dialog=0x920c240, icon=QMessageBox::Critical, text=..., strlist=..., ask=..., checkboxReturn=0x0, options=..., details=...)
    at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdeui/dialogs/kmessagebox.cpp:152
#12 0xb7434dcd in KMessageBox::errorListWId (parent_id=0, text=..., strlist=..., caption=..., options=...)
    at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdeui/dialogs/kmessagebox.cpp:847
#13 0xb7434f68 in KMessageBox::error (parent=0x0, text=..., caption=..., options=...)
    at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdeui/dialogs/kmessagebox.cpp:814
#14 0xb509bdce in MailCommon::Kernel::emergencyExit (this=0x90f7b40, reason=...) at /var/tmp/portage/kde-base/kmail-4.6.0-r1/work/kmail-4.6.0/mailcommon/mailkernel.cpp:193
#15 0xb509bea7 in MailCommon::Kernel::createDefaultCollectionDone (this=0x90f7b40, job=0x92c5bf0)
    at /var/tmp/portage/kde-base/kmail-4.6.0-r1/work/kmail-4.6.0/mailcommon/mailkernel.cpp:158
#16 0xb509c30a in MailCommon::Kernel::qt_metacall (this=0x90f7b40, _c=QMetaObject::InvokeMetaMethod, _id=6, _a=0xbfbc7058)
    at /var/tmp/portage/kde-base/kmail-4.6.0-r1/work/kmail-4.6.0_build/mailcommon/mailkernel.moc:82
#17 0xb61c8093 in QMetaObject::metacall (object=0x90f7b40, cl=QMetaObject::InvokeMetaMethod, idx=6, argv=0xbfbc7058) at kernel/qmetaobject.cpp:237
#18 0xb61db460 in QMetaObject::activate (sender=0x92c5bf0, m=0xb72d1528, local_signal_index=3, argv=0xfffffdfc) at kernel/qobject.cpp:3278
#19 0xb718ce94 in KJob::result (this=0x92c5bf0, _t1=0x92c5bf0) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3_build/kdecore/kjob.moc:194
#20 0xb718d274 in KJob::emitResult (this=0x92c5bf0) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdecore/jobs/kjob.cpp:312
#21 0xb5502292 in Akonadi::TransactionSequence::slotResult (this=0x92c5bf0, job=0x96bb4b8)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3/akonadi/transactionsequence.cpp:132
#22 0xb54ed4bc in Akonadi::SpecialCollectionsRequestJob::slotResult (this=0x92c5bf0, job=0x96bb4b8)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3/akonadi/specialcollectionsrequestjob.cpp:366
#23 0xb5501d7c in Akonadi::TransactionSequence::qt_metacall (this=0x92c5bf0, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xbfbc7308)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/transactionsequence.moc:76
#24 0xb54ed769 in Akonadi::SpecialCollectionsRequestJob::qt_metacall (this=0x92c5bf0, _c=QMetaObject::InvokeMetaMethod, _id=35, _a=0xbfbc7308)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/specialcollectionsrequestjob.moc:73
#25 0xb5363d73 in Akonadi::SpecialMailCollectionsRequestJob::qt_metacall (this=0x92c5bf0, _c=QMetaObject::InvokeMetaMethod, _id=35, _a=0xbfbc7308)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/kmime/specialmailcollectionsrequestjob.moc:64
#26 0xb61c8093 in QMetaObject::metacall (object=0x92c5bf0, cl=QMetaObject::InvokeMetaMethod, idx=35, argv=0xbfbc7308) at kernel/qmetaobject.cpp:237
#27 0xb61db460 in QMetaObject::activate (sender=0x96bb4b8, m=0xb72d1528, local_signal_index=3, argv=0xfffffdfc) at kernel/qobject.cpp:3278
#28 0xb718ce94 in KJob::result (this=0x96bb4b8, _t1=0x96bb4b8) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3_build/kdecore/kjob.moc:194
#29 0xb718d274 in KJob::emitResult (this=0x96bb4b8) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdecore/jobs/kjob.cpp:312
#30 0xb54e8fdf in Akonadi::DefaultResourceJobPrivate::collectionFetchResult (this=0x95b7d90, job=0x94d1ed0)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3/akonadi/specialcollectionshelperjobs.cpp:407
#31 0xb54eae3b in Akonadi::DefaultResourceJob::qt_metacall (this=0x96bb4b8, _c=QMetaObject::InvokeMetaMethod, _id=38, _a=0xbfbc75f8)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/specialcollectionshelperjobs_p.moc:140
#32 0xb61c8093 in QMetaObject::metacall (object=0x96bb4b8, cl=QMetaObject::InvokeMetaMethod, idx=38, argv=0xbfbc75f8) at kernel/qmetaobject.cpp:237
#33 0xb61db460 in QMetaObject::activate (sender=0x94d1ed0, m=0xb72d1528, local_signal_index=3, argv=0xfffffdfc) at kernel/qobject.cpp:3278
#34 0xb718ce94 in KJob::result (this=0x94d1ed0, _t1=0x94d1ed0) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3_build/kdecore/kjob.moc:194
#35 0xb718d274 in KJob::emitResult (this=0x94d1ed0) at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdecore/jobs/kjob.cpp:312
#36 0xb54a8a72 in Akonadi::JobPrivate::delayedEmitResult (this=0x9503b00) at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3/akonadi/job.cpp:144
#37 0xb54a8e14 in Akonadi::Job::qt_metacall (this=0x94d1ed0, _c=QMetaObject::InvokeMetaMethod, _id=6, _a=0x94f8d40)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/job.moc:91
#38 0xb5438365 in Akonadi::CollectionFetchJob::qt_metacall (this=0x94d1ed0, _c=QMetaObject::InvokeMetaMethod, _id=34, _a=0x94f8d40)
    at /var/tmp/portage/kde-base/kdepimlibs-4.6.3/work/kdepimlibs-4.6.3_build/akonadi/collectionfetchjob.moc:73
#39 0xb61c8093 in QMetaObject::metacall (object=0x94d1ed0, cl=QMetaObject::InvokeMetaMethod, idx=34, argv=0x94f8d40) at kernel/qmetaobject.cpp:237
#40 0xb61d2c7f in QMetaCallEvent::placeMetaCall (this=0x91fea28, object=0x94d1ed0) at kernel/qobject.cpp:535
#41 0xb61d3a2c in QObject::event (this=0x94d1ed0, e=0xfffffdfc) at kernel/qobject.cpp:1217
#42 0xb643b90f in QApplicationPrivate::notify_helper (this=0x905a0a8, receiver=0x94d1ed0, e=0x91fea28) at kernel/qapplication.cpp:4462
#43 0xb6443822 in QApplication::notify (this=0xbfbc7f8c, receiver=0x94d1ed0, e=0x91fea28) at kernel/qapplication.cpp:3862
#44 0xb74d7365 in KApplication::notify (this=0xbfbc7f8c, receiver=0x94d1ed0, event=0x91fea28)
    at /var/tmp/portage/kde-base/kdelibs-4.6.3-r2/work/kdelibs-4.6.3/kdeui/kernel/kapplication.cpp:311
#45 0xb61c0b25 in QCoreApplication::notifyInternal (this=0xbfbc7f8c, receiver=0x94d1ed0, event=0x91fea28) at kernel/qcoreapplication.cpp:731
#46 0xb61c20e0 in sendEvent (receiver=0x0, event_type=0, data=0x902b6e8) at kernel/qcoreapplication.h:215
#47 QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x902b6e8) at kernel/qcoreapplication.cpp:1372
#48 0xb61c238a in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1265
#49 0xb61f0fa3 in sendPostedEvents (s=0x9057100) at kernel/qcoreapplication.h:220
#50 postEventSourceDispatch (s=0x9057100) at kernel/qeventdispatcher_glib.cpp:277
#51 0xb45aeb60 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#52 0xb45b2c48 in ?? () from /usr/lib/libglib-2.0.so.0
#53 0xb45b2dfd in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#54 0xb61f0ba6 in QEventDispatcherGlib::processEvents (this=0x9052ae8, flags=...) at kernel/qeventdispatcher_glib.cpp:422
#55 0xb6500a2c in QGuiEventDispatcherGlib::processEvents (this=0x9052ae8, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#56 0xb61bf5ff in QEventLoop::processEvents (this=0xbfbc7ea4, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:149
#57 0xb61bfa25 in QEventLoop::exec (this=0xbfbc7ea4, flags=...) at kernel/qeventloop.cpp:201
#58 0xb61c2432 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1008
#59 0xb643a096 in QApplication::exec () at kernel/qapplication.cpp:3736
#60 0x0804a9ed in main (argc=1, argv=0xbfbc8084) at /var/tmp/portage/kde-base/kmail-4.6.0-r1/work/kmail-4.6.0/kmail/main.cpp:145