SUMMARY
I have tried opening may .ttf font files with kfontview, but no file could be opened by kfontview, but i know the files are not corrupt because kde and fc-list can detect and display them.


STEPS TO REPRODUCE
1. Download any .ttf font (in my case i had https://fonts.google.com/specimen/Open+Sans and some others and extracted it)
2. open kfontview directly via dolphin or open kfontview beforehand and use "open"
3. observe kfontview saying "Could not read font"

OBSERVED RESULT
Cannot open any .ttf font

EXPECTED RESULT
Be able to open any valid .ttf font

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.93.0
Qt Version: 5.15.3
Kernel Version: 5.17.6-1-MANJARO (64-bit)
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i7-7700K CPU @ 4.20GHz
Memory: 31.3 GiB of RAM
Graphics Processor: AMD Radeon RX Vega 64

ADDITIONAL INFORMATION
i have also tried opening some other google fonts and also tried to open a font listed by fc-list, but none could be read

i dont know how relevant it is, but i had also done a run via gdb and done a "bt" when the window came up:
#0  0x00007ffff67872af in poll () from /usr/lib/libc.so.6
#1  0x00007ffff52bb976 in g_main_context_poll (priority=<optimized out>, n_fds=1, fds=0x5555559ca970, timeout=<optimized out>, context=0x5555555ea330)
    at ../glib/glib/gmain.c:4516
#2  g_main_context_iterate.constprop.0 (context=context@entry=0x5555555ea330, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)
    at ../glib/glib/gmain.c:4206
#3  0x00007ffff52626c5 in g_main_context_iteration (context=0x5555555ea330, may_block=1) at ../glib/glib/gmain.c:4276
#4  0x00007ffff6d8957a in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#5  0x00007ffff6d3588b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#6  0x00007ffff79ae8b6 in QDialog::exec() () from /usr/lib/libQt5Widgets.so.5
#7  0x00007ffff6219819 in KMessageBox::createKMessageBox(QDialog*, QDialogButtonBox*, QIcon const&, QString const&, QStringList const&, QString const&, bool*, QFlags<KMessageBox::Option>, QString const&, QMessageBox::Icon) () from /usr/lib/libKF5WidgetsAddons.so.5
#8  0x00007ffff6219f15 in KMessageBox::createKMessageBox(QDialog*, QDialogButtonBox*, QMessageBox::Icon, QString const&, QStringList const&, QString const&, bool*, QFlags<KMessageBox::Option>, QString const&) () from /usr/lib/libKF5WidgetsAddons.so.5
#9  0x00007ffff6262f5c in ?? () from /usr/lib/libKF5WidgetsAddons.so.5
#10 0x00007ffff6216e74 in KMessageBox::error(QWidget*, QString const&, QString const&, QFlags<KMessageBox::Option>) ()
   from /usr/lib/libKF5WidgetsAddons.so.5
#11 0x00007ffff022d746 in ?? () from /usr/lib/qt/plugins/kf5/parts/kfontviewpart.so
#12 0x00007ffff6d6e463 in ?? () from /usr/lib/libQt5Core.so.5
#13 0x00007ffff022e378 in ?? () from /usr/lib/qt/plugins/kf5/parts/kfontviewpart.so
#14 0x00007ffff0230c71 in ?? () from /usr/lib/qt/plugins/kf5/parts/kfontviewpart.so
#15 0x00007ffff6d617d6 in QObject::event(QEvent*) () from /usr/lib/libQt5Core.so.5
#16 0x00007ffff77b42c6 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
#17 0x00007ffff6d3d5aa in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /usr/lib/libQt5Core.so.5
#18 0x00007ffff6d3e0a9 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/libQt5Core.so.5
#19 0x00007ffff6d85678 in ?? () from /usr/lib/libQt5Core.so.5
#20 0x00007ffff5265163 in g_main_dispatch (context=0x5555555ea330) at ../glib/glib/gmain.c:3417
#21 g_main_context_dispatch (context=0x5555555ea330) at ../glib/glib/gmain.c:4135
#22 0x00007ffff52bb9e9 in g_main_context_iterate.constprop.0 (context=context@entry=0x5555555ea330, block=block@entry=1, dispatch=dispatch@entry=1, 
    self=<optimized out>) at ../glib/glib/gmain.c:4211
#23 0x00007ffff52626c5 in g_main_context_iteration (context=0x5555555ea330, may_block=1) at ../glib/glib/gmain.c:4276
#24 0x00007ffff6d8957a in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#25 0x00007ffff6d3588b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#26 0x00007ffff6d40fd7 in QCoreApplication::exec() () from /usr/lib/libQt5Core.so.5
#27 0x000055555555b35b in ?? ()
#28 0x00007ffff66ae310 in __libc_start_call_main () from /usr/lib/libc.so.6
#29 0x00007ffff66ae3c1 in __libc_start_main_impl () from /usr/lib/libc.so.6
#30 0x000055555555b3d5 in ?? ()