SUMMARY
Unable to change username through KDE Settings.
After applying the change, the following error appears:

org.kde.kcm_users: "org.freedesktop.Accounts.Error.Failed" "running '/usr/sbin/usermod' failed: Child process exited with code 8"


STEPS TO REPRODUCE

1 Open System Settings → Users.
2 Select an existing user.
3 Change the username and apply changes.

OBSERVED RESULT
Username does not change. Console output shows the following log messages:

systemsettings 
qrc:/kcm/kcm_landingpage/main.qml:26:5: QML FormLayout: Binding loop detected for property "wideMode":
qrc:/qt/qml/org/kde/kirigami/layouts/FormLayout.qml:72:5
"The name is not activatable"
kf.kcmutils: Trying to use rootObject before initialization is completed, whilst using setInitializationDelayed. Forcing completion
qrc:/kcm/kcm_users/UserDetailsPage.qml:161:9: QML FormLayout: Binding loop detected for property "wideMode":
qrc:/qt/qml/org/kde/kirigami/layouts/FormLayout.qml:72:5
org.kde.kcm_users: "org.freedesktop.Accounts.Error.Failed" "running '/usr/sbin/usermod' failed: Child process exited with code 8"


EXPECTED RESULT
Changing the username via KDE Settings should complete successfully without errors.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20251110
KDE Plasma Version: 6.5.2
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0
Kernel Version: 6.17.7-1-default (64-bit)
Graphics Platform: Wayland
Processors: 4 × Intel® Xeon® CPU E3-1220 V2 @ 3.10GHz
Memory: 16 GiB (15.6 GiB usable)
Graphics Processor: AMD Radeon RX 570 Series

Also reproducible on:

Fedora 43 (KDE Plasma 6.4)

KDE Neon Stable (KDE Plasma 6.5.1)

ADDITIONAL INFORMATION
The issue consistently occurs on different distributions. The Users module seems to execute usermod without proper privileges or incorrect arguments.