SUMMARY
If I can provide more information, I will of course do as much as possible. For now I have this.

I am running Arch Linux and most likely due to the most recent package updates I got a new problem with KMyMoney. That is, after doing an account update the KMyMoney just shuts down (no crash dialog or something) after I close the "Tasks get executed ..." dialog where the AQBanking/HBCI messages are logged.
When I run KMyMoney from a shell, the last messages printed out are the following:

KMyMoneyPlugin::KMMStatementInterface::import start
malloc(): unaligned fastbin chunk detected
Abgebrochen (Speicherabzug geschrieben)

Or:

KMyMoneyPlugin::KMMStatementInterface::import start
free(): invalid next size (fast)
Abgebrochen (Speicherabzug geschrieben)

When I run KMyMoney under GDB and try to get a backtrace, I get this:
$ gdb kmymoney
...
...
Reading symbols from kmymoney...
(No debugging symbols found in kmymoney)
(gdb) run
Starting program: /usr/bin/kmymoney 
...
...
KMyMoneyPlugin::KMMStatementInterface::import start
free(): invalid next size (fast)

Thread 1 "kmymoney" received signal SIGABRT, Aborted.
0x00007ffff586d36c in ?? () from /usr/lib/libc.so.6
(gdb) bt
#0  0x00007ffff586d36c in  () at /usr/lib/libc.so.6
#1  0x00007ffff581d838 in raise () at /usr/lib/libc.so.6
#2  0x00007ffff5807535 in abort () at /usr/lib/libc.so.6
#3  0x00007ffff586145e in  () at /usr/lib/libc.so.6
#4  0x00007ffff58770cc in  () at /usr/lib/libc.so.6
#5  0x00007ffff587904a in  () at /usr/lib/libc.so.6
#6  0x00007ffff587b8f3 in free () at /usr/lib/libc.so.6
#7  0x00007ffff7cc0b68 in MyMoneyAccount::~MyMoneyAccount() () at /usr/lib/libkmm_mymoney.so.5
#8  0x00007ffff7c8effe in MyMoneyFile::modifyAccount(MyMoneyAccount const&) () at /usr/lib/libkmm_mymoney.so.5
#9  0x00005555557ea63b in  ()
#10 0x00005555557ec291 in  ()
#11 0x00005555557fd256 in  ()
#12 0x00007fffc82a1454 in  () at /usr/lib/qt/plugins/kmymoney/kbanking.so
#13 0x00007fffc829e3d4 in  () at /usr/lib/qt/plugins/kmymoney/kbanking.so
#14 0x00007fffc82a2456 in  () at /usr/lib/qt/plugins/kmymoney/kbanking.so
#15 0x00007fffc829c063 in  () at /usr/lib/qt/plugins/kmymoney/kbanking.so
#16 0x000055555565e95a in  ()
#17 0x000055555566295c in  ()
#18 0x00007ffff60e1791 in  () at /usr/lib/libQt5Core.so.5
#19 0x00007ffff6b97e07 in QAction::triggered(bool) () at /usr/lib/libQt5Widgets.so.5
#20 0x00007ffff6b9d817 in QAction::activate(QAction::ActionEvent) () at /usr/lib/libQt5Widgets.so.5
#21 0x00007ffff6d1f9db in  () at /usr/lib/libQt5Widgets.so.5
#22 0x00007ffff6d1fafd in  () at /usr/lib/libQt5Widgets.so.5
#23 0x00007ffff6bdb677 in QWidget::event(QEvent*) () at /usr/lib/libQt5Widgets.so.5
#24 0x00007ffff6ba4a7c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#25 0x00007ffff6baa289 in QApplication::notify(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#26 0x00007ffff60b0ff8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#27 0x00007ffff6ba8287 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) () at /usr/lib/libQt5Widgets.so.5
#28 0x00007ffff6bf9824 in  () at /usr/lib/libQt5Widgets.so.5
#29 0x00007ffff6bfb0ee in  () at /usr/lib/libQt5Widgets.so.5
#30 0x00007ffff6ba4a7c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#31 0x00007ffff60b0ff8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#32 0x00007ffff6504fdc in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) ()
    at /usr/lib/libQt5Gui.so.5
#33 0x00007ffff64ee945 in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt5Gui.so.5
#34 0x00007fffda0b7df0 in  () at /usr/lib/libQt5XcbQpa.so.5
#35 0x00007fffebb0a163 in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#36 0x00007fffebb609e9 in  () at /usr/lib/libglib-2.0.so.0
#37 0x00007fffebb076c5 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#38 0x00007ffff60fb7bc in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#39 0x00007ffff60a979c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#40 0x00007ffff60b42a9 in QCoreApplication::exec() () at /usr/lib/libQt5Core.so.5
#41 0x000055555562cc8d in  ()
#42 0x00007ffff5808290 in  () at /usr/lib/libc.so.6
#43 0x00007ffff580834a in __libc_start_main () at /usr/lib/libc.so.6
#44 0x000055555562e765 in  ()


STEPS TO REPRODUCE
1. select an account
2. use "Account -> Update Account"
3. wait for the update to complete and close the task logging dialog
4. after that KMyMoney disappears

OBSERVED RESULT
No new transactions get imported in the project file. KMyMoney closes unexpectedly.

EXPECTED RESULT
KMyMoney should not close down and new transactions should be imported to the project.

SOFTWARE/OS VERSIONS
KMyMoney Version 5.1.2
KDE Frameworks Version 5.94.0
Qt Version 5.15.4 (built against 5.15.3)
KDE Plasma Version: 5.24.5

ADDITIONAL INFORMATION