I encountered the same problem this morning on testing. I believe that
the crash is due to an incomplete upgrade of plasma.
/usr/lib/x86_64-linux-gnu/qt6/plugins/plasma/applets/org.kde.plasma.systemtray.so
is provided by plasma-workspace, which is version 4:6.3.6-2+b1 in
testing, but 4:6.5.0-2 in unstable. Upgrading to 4:6.5.0-2 fixed the
problem for me. (I wound up doing apt install -t unstable
plasma-desktop to make sure I upgraded everything.)
