I confirm it. There is still another bug report about it.
I tried to fix it but it very hard to report error to korganizer...

(In reply to Martin Koller from comment #0)
> Created attachment 122574 [details]
> test vcs
> 
> SUMMARY
> A webpage produced the attached vcs file.
> When importing it, korganizer shows a blue bar saying "1 incidence was
> successfully imported" but in konsole I see the log output:
> 
> Incidence has empty UID. id= 336260 ; summary= "Test" Please fix it.
> Ignoring this incidence.
> org.kde.pim.kcalcore: The incidence didn't have any UID! Report a bug  to
> the application that generated this file.
> 
> OBSERVED RESULT
> The incidence is not shown in korganizer
> 
> EXPECTED RESULT
> Show imported incidence.
> When an import finds a missing UID, it should simply generate one and import
> the file, instead of just ignoring this file. A user can not control what
> content a web server produces and there is zero chance that a user can
> convince any company to fix the producer.
> Please be more forgiving on broken input.
> 
> SOFTWARE/OS VERSIONS
> KDE Frameworks 5.61.0
> Qt 5.13.0 (built against 5.13.0)
> The xcb windowing system
> openSuse Leap 15.0

The mention of an issue with empty UID looks similar to https://bugs.kde.org/show_bug.cgi?id=378391#c2 , could you take a look?

can you test in the next frameworks release?

FTR: commit 
b3dc1e5d495721797dba0f28ee3eb01555bb5605 in KCalendarCore project

I tested now again with a self-compiled KDE-PIM (20.04) on

Operating System: openSUSE Tumbleweed 20200415
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.1
Kernel Version: 5.6.2-1-default
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-6300U CPU @ 2.40GHz
Memory: 15,5 GiB of RAM

Now korganizer and korgac crash when I import it and merge it into an existing calendar (stored on a nextcloud server).
backtrace of korganizer below.
When I import it as a new calendar, then it is still not shown in korganizer but at least no crash.
Checking with akonadiconsole and looking on the new calendar shows 0 items.

Thread 1 (Thread 0x7fd1985ed880 (LWP 15319)):
[KCrash Handler]
#6  0x00007fd19b572ea1 in raise () from /lib64/libc.so.6
#7  0x00007fd19b55c53d in abort () from /lib64/libc.so.6
#8  0x00007fd19bad1aef in qt_message_fatal (context=..., message=<synthetic pointer>...) at global/qlogging.cpp:1894
#9  QMessageLogger::fatal (this=this@entry=0x7fff2e279750, msg=msg@entry=0x7fd19bdbeea0 "ASSERT: \"%s\" in file %s, line %d") at global/qlogging.cpp:893
#10 0x00007fd19bad0f2f in qt_assert (assertion=<optimized out>, file=<optimized out>, line=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qlogging.h:90
#11 0x00007fd19e33ed39 in Akonadi::ETMCalendarPrivate::updateItem (this=0x2430480, item=...) at /home/KDE5/source/kde/pim/akonadi-calendar/src/etmcalendar.cpp:408
#12 0x00007fd19e33d534 in Akonadi::ETMCalendarPrivate::onDataChangedInFilteredModel (this=0x2430480, topLeft=..., bottomRight=...) at /home/KDE5/source/kde/pim/akonadi-calendar/src/etmcalendar.cpp:394
#13 0x00007fd19e34b947 in QtPrivate::FunctorCall<QtPrivate::IndexesList<0, 1>, QtPrivate::List<QModelIndex const&, QModelIndex const&>, void, void (Akonadi::ETMCalendarPrivate::*)(QModelIndex const&, QModelIndex const&)>::call (f=(void (Akonadi::ETMCalendarPrivate::*)(Akonadi::ETMCalendarPrivate * const, const QModelIndex &, const QModelIndex &)) 0x7fd19e33d420 <Akonadi::ETMCalendarPrivate::onDataChangedInFilteredModel(QModelIndex const&, QModelIndex const&)>, o=0x2430480, arg=0x7fff2e279bf0) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:152
#14 0x00007fd19e34b8a8 in QtPrivate::FunctionPointer<void (Akonadi::ETMCalendarPrivate::*)(QModelIndex const&, QModelIndex const&)>::call<QtPrivate::List<QModelIndex const&, QModelIndex const&>, void> (f=(void (Akonadi::ETMCalendarPrivate::*)(Akonadi::ETMCalendarPrivate * const, const QModelIndex &, const QModelIndex &)) 0x7fd19e33d420 <Akonadi::ETMCalendarPrivate::onDataChangedInFilteredModel(QModelIndex const&, QModelIndex const&)>, o=0x2430480, arg=0x7fff2e279bf0) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:185
#15 0x00007fd19e34b7d5 in QtPrivate::QSlotObject<void (Akonadi::ETMCalendarPrivate::*)(QModelIndex const&, QModelIndex const&), QtPrivate::List<QModelIndex const&, QModelIndex const&>, void>::impl (which=1, this_=0x246d9b0, r=0x2430480, a=0x7fff2e279bf0, ret=0x0) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:414
#16 0x00007fd19bd149fe in QtPrivate::QSlotObjectBase::call (a=0x7fff2e279bf0, r=0x2430480, this=0x246d9b0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:394
#17 doActivate<false> (sender=0x247a4b0, signal_index=3, argv=0x7fff2e279bf0) at kernel/qobject.cpp:3870
#18 0x00007fd19bd0f1bf in QMetaObject::activate (sender=sender@entry=0x247a4b0, m=m@entry=0x7fd19bfa1060 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff2e279bf0) at kernel/qobject.cpp:3930
#19 0x00007fd19bc83d35 in QAbstractItemModel::dataChanged (this=this@entry=0x247a4b0, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:557
#20 0x00007fd19bcb7b2d in QSortFilterProxyModelPrivate::_q_sourceDataChanged (this=0x247f6c0, source_top_left=..., source_bottom_right=..., roles=...) at itemmodels/qsortfilterproxymodel.cpp:1500
#21 0x00007fd19bcb8c2a in QSortFilterProxyModel::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at itemmodels/qsortfilterproxymodel.h:199
#22 0x00007fd19bd14a30 in doActivate<false> (sender=0x24430e0, signal_index=3, argv=0x7fff2e279ee0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:391
#23 0x00007fd19bd0f1bf in QMetaObject::activate (sender=sender@entry=0x24430e0, m=m@entry=0x7fd19bfa1060 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff2e279ee0) at kernel/qobject.cpp:3930
#24 0x00007fd19bc83d35 in QAbstractItemModel::dataChanged (this=this@entry=0x24430e0, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:557
#25 0x00007fd19bcb7b2d in QSortFilterProxyModelPrivate::_q_sourceDataChanged (this=0x247e6d0, source_top_left=..., source_bottom_right=..., roles=...) at itemmodels/qsortfilterproxymodel.cpp:1500
#26 0x00007fd19bcb8c2a in QSortFilterProxyModel::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at itemmodels/qsortfilterproxymodel.h:199
#27 0x00007fd19bd14a30 in doActivate<false> (sender=0x23fef30, signal_index=3, argv=0x7fff2e27a1d0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:391
#28 0x00007fd19bd0f1bf in QMetaObject::activate (sender=sender@entry=0x23fef30, m=m@entry=0x7fd19bfa1060 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff2e27a1d0) at kernel/qobject.cpp:3930
#29 0x00007fd19bc83d35 in QAbstractItemModel::dataChanged (this=this@entry=0x23fef30, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:557
#30 0x00007fd19bcb7b2d in QSortFilterProxyModelPrivate::_q_sourceDataChanged (this=0x247d3b0, source_top_left=..., source_bottom_right=..., roles=...) at itemmodels/qsortfilterproxymodel.cpp:1500
#31 0x00007fd19bcb8c2a in QSortFilterProxyModel::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at itemmodels/qsortfilterproxymodel.h:199
#32 0x00007fd19bd14a30 in doActivate<false> (sender=0x24947e0, signal_index=3, argv=0x7fff2e27a4c0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:391
#33 0x00007fd19bd0f1bf in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7fd19bfa1060 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff2e27a4c0) at kernel/qobject.cpp:3930
#34 0x00007fd19bc83d35 in QAbstractItemModel::dataChanged (this=<optimized out>, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:557
#35 0x00007fd19d94c07e in ?? () from /usr/lib64/libKF5ItemModels.so.5
#36 0x00007fd19bd14a30 in doActivate<false> (sender=0x2488730, signal_index=3, argv=0x7fff2e27a700) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:391
#37 0x00007fd19bd0f1bf in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7fd19bfa1060 <QAbstractItemModel::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff2e27a700) at kernel/qobject.cpp:3930
#38 0x00007fd19bc83d35 in QAbstractItemModel::dataChanged (this=<optimized out>, _t1=..., _t2=..., _t3=...) at .moc/moc_qabstractitemmodel.cpp:557
#39 0x00007fd19db89241 in Akonadi::EntityTreeModelPrivate::dataChanged (this=0x2449b50, top=..., bottom=...) at /home/KDE5/source/kde/pim/akonadi/src/core/models/entitytreemodel_p.cpp:1721
#40 0x00007fd19db8e298 in Akonadi::EntityTreeModelPrivate::monitoredItemChanged (this=0x2449b50, item=...) at /home/KDE5/source/kde/pim/akonadi/src/core/models/entitytreemodel_p.cpp:1186
#41 0x00007fd19db80954 in Akonadi::EntityTreeModel::qt_static_metacall (_o=0x2488730, _c=QMetaObject::InvokeMetaMethod, _id=26, _a=0x7fff2e27ab80) at KF5AkonadiCore_autogen/include/moc_entitytreemodel.cpp:235
#42 0x00007fd19bd14a30 in doActivate<false> (sender=0x23ce3d0, signal_index=3, argv=0x7fff2e27ab80) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:391
#43 0x00007fd19da86d57 in Akonadi::Monitor::itemChanged (this=0x23ce3d0, _t1=..., _t2=...) at KF5AkonadiCore_autogen/include/moc_monitor.cpp:902
#44 0x00007fd19da9fce9 in Akonadi::MonitorPrivate::emitToListeners<void (Akonadi::Monitor::*)(Akonadi::Item const&, QSet<QByteArray> const&), Akonadi::Item, QSet<QByteArray> > (this=0x24915f0, signal=(void (Akonadi::Monitor::*)(Akonadi::Monitor * const, const Akonadi::Item &, const QSet<QByteArray> &)) 0x7fd19da86d00 <Akonadi::Monitor::itemChanged(Akonadi::Item const&, QSet<QByteArray> const&)>, args=..., args=...) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.h:404
#45 0x00007fd19da9571d in Akonadi::MonitorPrivate::emitItemsNotification (this=0x24915f0, msg_=..., items=..., collection=..., collectionDest=...) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.cpp:1069
#46 0x00007fd19da93045 in Akonadi::MonitorPrivate::emitNotification (this=0x24915f0, msg=...) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.cpp:558
#47 0x00007fd19da99cd7 in Akonadi::MonitorPrivate::dispatchNotifications (this=0x24915f0) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.cpp:1003
#48 0x00007fd19da99786 in Akonadi::MonitorPrivate::slotNotify (this=0x24915f0, msg=...) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.cpp:972
#49 0x00007fd19da98e14 in Akonadi::MonitorPrivate::handleCommands (this=0x24915f0) at /home/KDE5/source/kde/pim/akonadi/src/core/monitor_p.cpp:867
#50 0x00007fd19da8570f in Akonadi::Monitor::qt_static_metacall (_o=0x23ce3d0, _c=QMetaObject::InvokeMetaMethod, _id=42, _a=0x7fd17c0486b8) at KF5AkonadiCore_autogen/include/moc_monitor.cpp:339
#51 0x00007fd19bd0ca19 in QObject::event (this=0x23ce3d0, e=0x7fd17c048670) at kernel/qobject.cpp:1339
#52 0x00007fd19c8ddcaf in QApplicationPrivate::notify_helper (this=this@entry=0x22916b0, receiver=receiver@entry=0x23ce3d0, e=e@entry=0x7fd17c048670) at kernel/qapplication.cpp:3684
#53 0x00007fd19c8e6df0 in QApplication::notify (this=0x7fff2e27c468, receiver=0x23ce3d0, e=0x7fd17c048670) at kernel/qapplication.cpp:3430
#54 0x00007fd19bce0002 in QCoreApplication::notifyInternal2 (receiver=0x23ce3d0, event=0x7fd17c048670) at ../../include/QtCore/../../src/corelib/kernel/qobject.h:153
#55 0x00007fd19bce2794 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x228d120) at kernel/qcoreapplication.cpp:1832
#56 0x00007fd19bd37573 in postEventSourceDispatch (s=0x23527f0) at kernel/qeventdispatcher_glib.cpp:277
#57 0x00007fd199915048 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#58 0x00007fd1999153d0 in ?? () from /usr/lib64/libglib-2.0.so.0
#59 0x00007fd19991545f in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#60 0x00007fd19bd36bee in QEventDispatcherGlib::processEvents (this=0x2368680, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#61 0x00007fd19bcdeb9b in QEventLoop::exec (this=this@entry=0x7fff2e27c350, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:136
#62 0x00007fd19bce6972 in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:118
#63 0x000000000040a2a2 in main (argc=1, argv=0x7fff2e27c588) at /home/KDE5/source/kde/pim/korganizer/src/main.cpp:64

*** This bug has been marked as a duplicate of bug 339726 ***