Looks like the same backtrace as Bug 442222, which was supposed to have been fixed in Plasma 5.27.

Do you have the "Dim Inactive" effect enabled, and are you closing a grouped app?

Yes, Dim Inactive effect is in effect ;) though I definitely didn't use audacity for some weeks.

Anyway, here is another backtrace registered in coredumpctl ... it also happened on unhiding the panel (but this time for the highlight window effect - similar symptom, but totally different stacktrace):

(gdb) bt
#0  __GI___pthread_sigmask (how=1, newmask=<optimized out>, oldmask=0x0)
    at ./nptl/pthread_sigmask.c:43
#1  0x00007f278e23c6ad in __GI___sigprocmask (how=<optimized out>, set=<optimized out>, 
    oset=<optimized out>) at ../sysdeps/unix/sysv/linux/sigprocmask.c:25
#2  0x00007f27913f1fcf in KCrash::setCrashHandler (handler=handler@entry=0x0)
    at ./src/kcrash.cpp:414
#3  0x00007f27913f44fb in KCrash::defaultCrashHandler (sig=11) at ./src/kcrash.cpp:625
#4  <signal handler called>
#5  0x0000563153dc9e99 in KWin::isHighlightWindow (window=0x563155ea2290)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/src/effects/highlightwindow/highlightwindow.cpp:57
#6  KWin::HighlightWindowEffect::finishHighlighting (this=this@entry=0x563156acc200)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/src/effects/highlightwindow/highlightwindow.cpp:178
#7  0x0000563153dcb168 in KWin::HighlightWindowEffect::highlightWindows (windows=..., 
    this=0x563156acc200)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/src/effects/highlightwindow/highlightwindow.cpp:198
#8  KWin::HighlightWindowEffect::highlightWindows (windows=..., this=0x563156acc200)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/src/effects/highlightwindow/highlightwindow.cpp:71
#9  KWin::HighlightWindowEffect::qt_static_metacall (_o=0x563156acc200, 
    _c=<optimized out>, _id=<optimized out>, _a=<optimized out>)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/obj-x86_64-linux-gnu/src/effects/highlightwindow/kwin4_effect_highlightwindow_autogen/EWIEGA46WW/moc_highlightwindow.cpp:105
#10 0x0000563153dcb243 in KWin::HighlightWindowEffect::qt_metacall (this=0x563156acc200, 
    _c=QMetaObject::InvokeMetaMethod, _id=5, _a=0x7ffcd0a0c2b0)
    at /usr/src/kwin-4:5.27.4-0ubuntu1/obj-x86_64-linux-gnu/src/effects/highlightwindow/kwin4_effect_highlightwindow_autogen/EWIEGA46WW/moc_highlightwindow.cpp:182
#11 0x00007f278e8fa22b in QDBusConnectionPrivate::deliverCall (
    this=this@entry=0x7f27680013f0, object=object@entry=0x563156acc200, msg=..., 
    metaTypes=..., slotIdx=<optimized out>)
    at ../../include/QtCore/../../src/corelib/tools/qvarlengtharray.h:189
#12 0x00007f278e8fdf86 in QDBusConnectionPrivate::activateCall (
    this=this@entry=0x7f27680013f0, object=0x563156acc200, flags=240, msg=...)
    at qdbusintegrator.cpp:911
#13 0x00007f278e8fe55e in QDBusConnectionPrivate::activateCall (msg=..., 
    flags=<optimized out>, object=<optimized out>, this=0x7f27680013f0)
    at qdbusintegrator.cpp:853
#14 QDBusConnectionPrivate::activateObject (this=0x7f27680013f0, node=..., msg=..., 
    pathStartPos=<optimized out>) at qdbusintegrator.cpp:1521
#15 0x00007f278e9009fc in QDBusActivateObjectEvent::placeMetaCall (this=0x56315641cdb0)
    at qdbusintegrator.cpp:1617
#16 0x00007f278f6e7de0 in QObject::event (this=0x563156acc200, e=0x56315641cdb0)
    at kernel/qobject.cpp:1347
#17 0x00007f278eb6bf92 in QApplicationPrivate::notify_helper (this=<optimized out>, 
    receiver=0x563156acc200, e=0x56315641cdb0) at kernel/qapplication.cpp:3640
#18 0x00007f278f6bae38 in QCoreApplication::notifyInternal2 (receiver=0x563156acc200, 
    event=0x56315641cdb0) at kernel/qcoreapplication.cpp:1064
#19 0x00007f278f6bb012 in QCoreApplication::sendEvent (receiver=<optimized out>, 
    event=<optimized out>) at kernel/qcoreapplication.cpp:1462
--Type <RET> for more, q to quit, c to continue without paging--c
#20 0x00007f278f6bdea1 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x5631556e6580) at kernel/qcoreapplication.cpp:1821
#21 0x00007f278f6be32c in QCoreApplication::sendPostedEvents (receiver=<optimized out>, event_type=<optimized out>) at kernel/qcoreapplication.cpp:1680
#22 0x00007f278f7157b7 in postEventSourceDispatch (s=0x5631557d2740) at kernel/qeventdispatcher_glib.cpp:277
#23 0x00007f278ce6149d in g_main_dispatch (context=0x7f2784000ee0) at ../../../glib/gmain.c:3460
#24 g_main_context_dispatch (context=0x7f2784000ee0) at ../../../glib/gmain.c:4200
#25 0x00007f278cebc178 in g_main_context_iterate.constprop.0 (context=0x7f2784000ee0, block=<optimized out>, dispatch=1, self=<optimized out>) at ../../../glib/gmain.c:4276
#26 0x00007f278ce601b0 in g_main_context_iteration (context=0x7f2784000ee0, may_block=1) at ../../../glib/gmain.c:4343
#27 0x00007f278f714e7a in QEventDispatcherGlib::processEvents (this=0x5631557e43c0, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#28 0x00007f278f6b97cb in QEventLoop::exec (this=this@entry=0x7ffcd0a0c870, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:69
#29 0x00007f278f6c1c1a in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#30 0x00007f278fb35db0 in QGuiApplication::exec () at kernel/qguiapplication.cpp:1863
#31 0x00007f278eb6bf09 in QApplication::exec () at kernel/qapplication.cpp:2832
#32 0x0000563153d98b65 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/kwin-4:5.27.4-0ubuntu1/src/main_x11.cpp:447
(gdb)

> and are you closing a grouped app?

Happened again - same stack-trace. There is definitely no mouse-click or keyboard-input involved. I only move the pointer to uncover the auto-hide-panel (usually over the area of the task-list or the virtual desktop switcher). As either of the stack-traces only happens about once a day, I more like suspect a race condition of the drawing code not finishing in time before the mouse moves somewhere else and forces e.g. a hover effect on the unfinished drawing ...

*** This bug has been marked as a duplicate of bug 468067 ***