Hello again, I also found the applicable upstream KDE bug:
"Akonadi fails with Mariadb 10.6.3" https://bugs.kde.org/show_bug.cgi?id=439769 Meanwhile, applying the fix suggested for Qt 5.15 in a slightly modified form seems to prevent the error I experienced from occurring. With the modified libqt5sql5-mysql package installed, I have managed to start Kontact, view stored messages (after some messing around with akonadiconsole, due to earlier troubleshooting attempts), and this message will have been sent from Kontact. I conclude, then, that the upstream fix needs backporting to Qt 5.11.3 for Debian. Paul
