Doesn't crash here, if you can't provide a backtrace (the report from drkonqui) we really can't do anything.

Created attachment 93687 [details]
backtrace

experimentally i enabled AIGLX and Composite today and the instant crashes on hover seem to be gone.
on the other hand, the ui now lags sometimes where it did not previously, due apparently to fancy effects i don't want (which is, why both were disabled in the first place).

Thanks,
Note than you can disable fancy effects without disabling compositing. 

However, in any case we shouldn't crash at all, ever. So I'll try and look into this.

#31 0x00007fd404b61c15 in __GI_exit (status=<optimized out>) at exit.c:104
#32 0x00007fd3f73481de in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqxcb.so

This is an interesting bit; something in the X backend seems to be telling us to quit(?!). Is there any chance you could install debug symbols for the qt xcb backend, so I can see what function that is. Sorry I don't know what the exact package name will be for your distro.

Created attachment 93724 [details]
new backtrace with additional -dbg packages installed

Created attachment 93858 [details]
additional plasmashell kcrash logs

"attach" discarded my comment: i don't know if those kcrash logs are related, but the always end in the same line, do not happen anymore on hover, but rather often and most reliable after saving the kcrash log and when closing the kcrash dialog.

pasting inline

Thread 1 (Thread 0x7f0c62a6e900 (LWP 4397)):
[KCrash Handler]
#6  0x00007f0c6033c1b9 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#7  0x00007f0c605a50c8 in QQmlDelegateModel::_q_itemsChanged(int, int, QVector<int> const&) () from /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f0c605ccd2c in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#9  0x00007f0c605cd46f in QQmlDelegateModel::qt_metacall(QMetaObject::Call, int, void**) () from /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#10 0x00007f0c5def623a in QMetaObject::activate (sender=0x3058660, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f2d20) at kernel/qobject.cpp:3732
#11 0x00007f0c5def67a7 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7f0c5e102a80 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f2d20) at kernel/qobject.cpp:3582
#12 0x00007f0c5df7383c in QAbstractItemModel::dataChanged (this=<optimized out>, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:393
#13 0x00007f0b655fedb4 in TaskManager::TasksModelPrivate::itemChanged (this=0x3062210, changes=...) at ../../libtaskmanager/tasksmodel.cpp:521
#14 0x00007f0b655ff7a7 in TaskManager::TasksModel::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=0x7ffc889f2f10) at ./moc_tasksmodel.cpp:213
#15 0x00007f0c5def612e in QMetaObject::activate (sender=0x2ed7e20, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f2f10) at kernel/qobject.cpp:3717
#16 0x00007f0c5def67a7 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7f0b65824960 <TaskManager::AbstractGroupableItem::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f2f10) at kernel/qobject.cpp:3582
#17 0x00007f0b65607748 in TaskManager::AbstractGroupableItem::changed (this=<optimized out>, _t1=...) at moc_abstractgroupableitem.cpp:222
#18 0x00007f0b65608b7b in TaskManager::AbstractGroupableItem::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at moc_abstractgroupableitem.cpp:134
#19 0x00007f0c5def612e in QMetaObject::activate (sender=sender@entry=0x43d0d80, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f30a0) at kernel/qobject.cpp:3717
#20 0x00007f0c5def67a7 in QMetaObject::activate (sender=sender@entry=0x43d0d80, m=m@entry=0x7f0b65824660 <TaskManager::Task::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7ffc889f30a0) at kernel/qobject.cpp:3582
#21 0x00007f0b65607938 in TaskManager::Task::changed (this=this@entry=0x43d0d80, _t1=...) at moc_task.cpp:354
#22 0x00007f0b655e81ea in TaskManager::Task::setActive (this=this@entry=0x43d0d80, a=a@entry=true) at ../../libtaskmanager/task.cpp:178
#23 0x00007f0b655fad9c in TaskManager::TaskManager::activeWindowChanged (this=0x7f0b65826830 <TaskManager::(anonymous namespace)::Q_QGS_privateTaskManagerSelf::innerFunction()::holder>, w=<optimized out>) at ../../libtaskmanager/taskmanager.cpp:412
#24 0x00007f0b655fd119 in TaskManager::TaskManager::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at ./moc_taskmanager.cpp:175
#25 0x00007f0c5def612e in QMetaObject::activate (sender=0x7f0c6028b170, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=0x7ffc889f3290) at kernel/qobject.cpp:3717
#26 0x00007f0c60079832 in KWindowSystem::activeWindowChanged(unsigned long long) () from /usr/lib/x86_64-linux-gnu/libKF5WindowSystem.so.5
#27 0x00007f0c60062506 in ?? () from /usr/lib/x86_64-linux-gnu/libKF5WindowSystem.so.5
#28 0x00007f0c5dec1c90 in QAbstractEventDispatcher::filterNativeEvent (this=this@entry=0x1805ea0, eventType=..., message=message@entry=0x7f0c480168b0, result=result@entry=0x7ffc889f33f8) at kernel/qabstracteventdispatcher.cpp:460
#29 0x00007f0c4fd99a3d in QXcbConnection::handleXcbEvent (this=this@entry=0x17c7c10, event=event@entry=0x7f0c480168b0) at qxcbconnection.cpp:863
#30 0x00007f0c4fd9aecb in QXcbConnection::processXcbEvents (this=0x17c7c10) at qxcbconnection.cpp:1303
#31 0x00007f0c5def6e8a in QObject::event (this=0x17c7c10, e=<optimized out>) at kernel/qobject.cpp:1245
#32 0x00007f0c5e9c3adc in QApplicationPrivate::notify_helper (this=this@entry=0x17bd2e0, receiver=receiver@entry=0x17c7c10, e=e@entry=0x7f0c48015350) at kernel/qapplication.cpp:3720
#33 0x00007f0c5e9c8fb0 in QApplication::notify (this=0x7ffc889f3c70, receiver=0x17c7c10, e=0x7f0c48015350) at kernel/qapplication.cpp:3503
#34 0x00007f0c5dec53ab in QCoreApplication::notifyInternal (this=0x7ffc889f3c70, receiver=0x17c7c10, event=event@entry=0x7f0c48015350) at kernel/qcoreapplication.cpp:935
#35 0x00007f0c5dec741b in sendEvent (event=0x7f0c48015350, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:228
#36 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x17b2bb0) at kernel/qcoreapplication.cpp:1552
#37 0x00007f0c5dec7a18 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1410
#38 0x00007f0c5df1df33 in postEventSourceDispatch (s=0x180beb0) at kernel/qeventdispatcher_glib.cpp:271
#39 0x00007f0c5a1e6c3d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#40 0x00007f0c5a1e6f20 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#41 0x00007f0c5a1e6fcc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#42 0x00007f0c5df1e347 in QEventDispatcherGlib::processEvents (this=0x1805ea0, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#43 0x00007f0c5dec2b62 in QEventLoop::exec (this=this@entry=0x7ffc889f3af0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#44 0x00007f0c5deca7ac in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1188
#45 0x00007f0c5e1ed1fc in QGuiApplication::exec () at kernel/qguiapplication.cpp:1507
#46 0x00007f0c5e9bf745 in QApplication::exec () at kernel/qapplication.cpp:2956
#47 0x0000000000430a43 in main (argc=2, argv=<optimized out>) at ../../shell/main.cpp:154

*** This bug has been marked as a duplicate of bug 342763 ***