Here is a backtrace, I hope it is complete enough...: 
Application: akonadi_davgroupware_resource (akonadi_davgroupware_resource), signal: Segmentation fault
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f53c21d1900 (LWP 13082))]

Thread 5 (Thread 0x7f53877fe700 (LWP 13344)):
#0  0x00007f53bcadb37d in read () from /usr/lib/libc.so.6
#1  0x00007f53b274ee20 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007f53b270a47e in g_main_context_check () from /usr/lib/libglib-2.0.so.0
#3  0x00007f53b270a954 in ?? () from /usr/lib/libglib-2.0.so.0
#4  0x00007f53b270aacc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#5  0x00007f53bd61f10b in QEventDispatcherGlib::processEvents (this=0x7f53780008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#6  0x00007f53bd5c893a in QEventLoop::exec (this=this@entry=0x7f53877fdcb0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#7  0x00007f53bd3eaae3 in QThread::exec (this=<optimized out>) at thread/qthread.cpp:507
#8  0x00007f53bd3ef748 in QThreadPrivate::start (arg=0x1890520) at thread/qthread_unix.cpp:368
#9  0x00007f53b655c2e7 in start_thread () from /usr/lib/libpthread.so.0
#10 0x00007f53bcae954f in clone () from /usr/lib/libc.so.6

Thread 4 (Thread 0x7f5387fff700 (LWP 13290)):
#0  0x00007f53b27501c9 in g_mutex_lock () from /usr/lib/libglib-2.0.so.0
#1  0x00007f53b270a972 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007f53b270aacc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#3  0x00007f53bd61f10b in QEventDispatcherGlib::processEvents (this=0x7f53800008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#4  0x00007f53bd5c893a in QEventLoop::exec (this=this@entry=0x7f5387ffecb0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#5  0x00007f53bd3eaae3 in QThread::exec (this=<optimized out>) at thread/qthread.cpp:507
#6  0x00007f53bd3ef748 in QThreadPrivate::start (arg=0x18846c0) at thread/qthread_unix.cpp:368
#7  0x00007f53b655c2e7 in start_thread () from /usr/lib/libpthread.so.0
#8  0x00007f53bcae954f in clone () from /usr/lib/libc.so.6

Thread 3 (Thread 0x7f538ce61700 (LWP 13283)):
#0  0x00007f53bcadb37d in read () from /usr/lib/libc.so.6
#1  0x00007f53b274ee20 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007f53b270a47e in g_main_context_check () from /usr/lib/libglib-2.0.so.0
#3  0x00007f53b270a954 in ?? () from /usr/lib/libglib-2.0.so.0
#4  0x00007f53b270aacc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#5  0x00007f53bd61f10b in QEventDispatcherGlib::processEvents (this=0x7f53880008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#6  0x00007f53bd5c893a in QEventLoop::exec (this=this@entry=0x7f538ce60c80, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#7  0x00007f53bd3eaae3 in QThread::exec (this=this@entry=0x7f53be8bad60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at thread/qthread.cpp:507
#8  0x00007f53be646835 in QDBusConnectionManager::run (this=0x7f53be8bad60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at qdbusconnection.cpp:170
#9  0x00007f53bd3ef748 in QThreadPrivate::start (arg=0x7f53be8bad60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at thread/qthread_unix.cpp:368
#10 0x00007f53b655c2e7 in start_thread () from /usr/lib/libpthread.so.0
#11 0x00007f53bcae954f in clone () from /usr/lib/libc.so.6

Thread 2 (Thread 0x7f5395f36700 (LWP 13193)):
#0  0x00007f53bcadf67d in poll () from /usr/lib/libc.so.6
#1  0x00007f53b69828e0 in ?? () from /usr/lib/libxcb.so.1
#2  0x00007f53b6984679 in xcb_wait_for_event () from /usr/lib/libxcb.so.1
#3  0x00007f5397e239f9 in QXcbEventReader::run (this=0x13b2e10) at qxcbconnection.cpp:1345
#4  0x00007f53bd3ef748 in QThreadPrivate::start (arg=0x13b2e10) at thread/qthread_unix.cpp:368
#5  0x00007f53b655c2e7 in start_thread () from /usr/lib/libpthread.so.0
#6  0x00007f53bcae954f in clone () from /usr/lib/libc.so.6

Thread 1 (Thread 0x7f53c21d1900 (LWP 13082)):
[KCrash Handler]
#6  QHash<QString, QHashDummyValue>::isEmpty (this=this@entry=0x18) at /usr/include/qt/QtCore/qhash.h:267
#7  QHash<QString, QHashDummyValue>::remove (this=this@entry=0x18, akey=...) at /usr/include/qt/QtCore/qhash.h:787
#8  0x00007f53c19b668d in QSet<QString>::remove (value=..., this=0x18) at /usr/include/qt/QtCore/qset.h:94
#9  KDAV::EtagCache::removeEtag (this=0x0, remoteId=...) at /home/jr/src/kdav/src/kdav-17.04.0/src/common/etagcache.cpp:68
#10 0x0000000000450b7e in DavGroupwareResource::onItemChangedFinished (this=0x1884200, job=<optimized out>) at /home/jr/src/kdepim-runtime/src/kdepim-runtime-17.04.0/resources/dav/resource/davgroupwareresource.cpp:1067
#11 0x00007f53bd5f615e in QtPrivate::QSlotObjectBase::call (a=0x7ffd4a41f110, r=0x1884200, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:101
#12 QMetaObject::activate (sender=sender@entry=0x19e0310, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f110) at kernel/qobject.cpp:3723
#13 0x00007f53bd5f6707 in QMetaObject::activate (sender=sender@entry=0x19e0310, m=m@entry=0x7f53be62d500 <KJob::staticMetaObject>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f110) at kernel/qobject.cpp:3602
#14 0x00007f53be3d63f2 in KJob::result (this=this@entry=0x19e0310, _t1=<optimized out>, _t1@entry=0x19e0310) at /home/jr/src/kcoreaddons/src/build/src/lib/KF5CoreAddons_autogen/include/moc_kjob.cpp:570
#15 0x00007f53be3d7d61 in KJob::finishJob (this=this@entry=0x19e0310, emitResult=emitResult@entry=true) at /home/jr/src/kcoreaddons/src/kcoreaddons-5.33.0/src/lib/jobs/kjob.cpp:109
#16 0x00007f53be3d9d2a in KJob::emitResult (this=this@entry=0x19e0310) at /home/jr/src/kcoreaddons/src/kcoreaddons-5.33.0/src/lib/jobs/kjob.cpp:293
#17 0x00007f53c19a5ae2 in KDAV::DavItemModifyJob::itemRefreshed (this=0x19e0310, job=<optimized out>) at /home/jr/src/kdav/src/kdav-17.04.0/src/common/davitemmodifyjob.cpp:131
#18 0x00007f53bd5f615e in QtPrivate::QSlotObjectBase::call (a=0x7ffd4a41f2c0, r=0x19e0310, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:101
#19 QMetaObject::activate (sender=sender@entry=0x19a7610, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f2c0) at kernel/qobject.cpp:3723
#20 0x00007f53bd5f6707 in QMetaObject::activate (sender=sender@entry=0x19a7610, m=m@entry=0x7f53be62d500 <KJob::staticMetaObject>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f2c0) at kernel/qobject.cpp:3602
#21 0x00007f53be3d63f2 in KJob::result (this=this@entry=0x19a7610, _t1=<optimized out>, _t1@entry=0x19a7610) at /home/jr/src/kcoreaddons/src/build/src/lib/KF5CoreAddons_autogen/include/moc_kjob.cpp:570
#22 0x00007f53be3d7d61 in KJob::finishJob (this=this@entry=0x19a7610, emitResult=emitResult@entry=true) at /home/jr/src/kcoreaddons/src/kcoreaddons-5.33.0/src/lib/jobs/kjob.cpp:109
#23 0x00007f53be3d9d2a in KJob::emitResult (this=this@entry=0x19a7610) at /home/jr/src/kcoreaddons/src/kcoreaddons-5.33.0/src/lib/jobs/kjob.cpp:293
#24 0x00007f53c19a5257 in KDAV::DavItemFetchJob::davJobFinished (this=0x19a7610, job=<optimized out>) at /home/jr/src/kdav/src/kdav-17.04.0/src/common/davitemfetchjob.cpp:87
#25 0x00007f53bd5f615e in QtPrivate::QSlotObjectBase::call (a=0x7ffd4a41f4f0, r=0x19a7610, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:101
#26 QMetaObject::activate (sender=sender@entry=0x19ab700, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f4f0) at kernel/qobject.cpp:3723
#27 0x00007f53bd5f6707 in QMetaObject::activate (sender=sender@entry=0x19ab700, m=m@entry=0x7f53be62d500 <KJob::staticMetaObject>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0x7ffd4a41f4f0) at kernel/qobject.cpp:3602
#28 0x00007f53be3d63f2 in KJob::result (this=this@entry=0x19ab700, _t1=<optimized out>, _t1@entry=0x19ab700) at /home/jr/src/kcoreaddons/src/build/src/lib/KF5CoreAddons_autogen/include/moc_kjob.cpp:570
#29 0x00007f53be3d7d61 in KJob::finishJob (this=0x19ab700, emitResult=<optimized out>) at /home/jr/src/kcoreaddons/src/kcoreaddons-5.33.0/src/lib/jobs/kjob.cpp:109
#30 0x00007f53bf10e1d2 in KIO::SimpleJob::slotFinished() () from /usr/lib/libKF5KIOCore.so.5
#31 0x00007f53bf11abd6 in KIO::TransferJob::slotFinished() () from /usr/lib/libKF5KIOCore.so.5
#32 0x00007f53bf11b5b1 in ?? () from /usr/lib/libKF5KIOCore.so.5
#33 0x00007f53bd5f5de9 in QMetaObject::activate (sender=0x18d5d20, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3740
#34 0x00007f53bf0e7e01 in KIO::SlaveInterface::dispatch(int, QByteArray const&) () from /usr/lib/libKF5KIOCore.so.5
#35 0x00007f53bf0e6447 in KIO::SlaveInterface::dispatch() () from /usr/lib/libKF5KIOCore.so.5
#36 0x00007f53bf0eb621 in KIO::Slave::gotInput() () from /usr/lib/libKF5KIOCore.so.5
#37 0x00007f53bf189635 in ?? () from /usr/lib/libKF5KIOCore.so.5
#38 0x00007f53bd5f5de9 in QMetaObject::activate (sender=0x18bece0, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3740
#39 0x00007f53bf07eff0 in ?? () from /usr/lib/libKF5KIOCore.so.5
#40 0x00007f53bd5f6c49 in QObject::event (this=0x18bece0, e=<optimized out>) at kernel/qobject.cpp:1263
#41 0x00007f53bf74f3dc in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x18bece0, e=0x190ce70) at kernel/qapplication.cpp:3745
#42 0x00007f53bf756bf1 in QApplication::notify (this=0x7ffd4a4201d0, receiver=0x18bece0, e=0x190ce70) at kernel/qapplication.cpp:3502
#43 0x00007f53bd5ca4e0 in QCoreApplication::notifyInternal2 (receiver=0x18bece0, event=event@entry=0x190ce70) at kernel/qcoreapplication.cpp:988
#44 0x00007f53bd5ccc6d in QCoreApplication::sendEvent (event=0x190ce70, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#45 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x1393ee0) at kernel/qcoreapplication.cpp:1648
#46 0x00007f53bd5cd0d8 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1502
#47 0x00007f53bd61ece3 in postEventSourceDispatch (s=0x13d56e0) at kernel/qeventdispatcher_glib.cpp:276
#48 0x00007f53b270a7b7 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#49 0x00007f53b270aa20 in ?? () from /usr/lib/libglib-2.0.so.0
#50 0x00007f53b270aacc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#51 0x00007f53bd61f0ef in QEventDispatcherGlib::processEvents (this=0x13deb30, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#52 0x00007f53bd5c893a in QEventLoop::exec (this=this@entry=0x7ffd4a420130, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#53 0x00007f53bd5d0e84 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1261
#54 0x00007f53c1c02bbe in Akonadi::ResourceBase::init(Akonadi::ResourceBase*) () from /usr/lib/libKF5AkonadiAgentBase.so.5
#55 0x000000000045ba87 in Akonadi::ResourceBase::init<DavGroupwareResource> (argc=<optimized out>, argv=<optimized out>) at /usr/include/KF5/AkonadiAgentBase/resourcebase.h:196
#56 0x00007f53bca1d511 in __libc_start_main () from /usr/lib/libc.so.6
#57 0x00000000004178aa in _start ()