Hello David Erwan,
this sounds like it could be a duplicate of bug #907301 [1].
Is your systray still visible?
If not you can check if this is really your
problem by starting this in a terminal:
QT_LOGGING_RULES="*plasma*=true" plasmawindowed org.kde.plasma.systemtray
And you receive output containing this:
... this plugin is compiled against incompatible Plasma version 340224 This
build is compatible ...
As a workaround you might try to rebuild plasma-workspace
locally and install the resulting plasma-workspace_*.deb
(Like reported in [2] should that package be enough.)
Another user reported that he could work around the issue
by installing two packages from unstable [3].
Kind regards,
Bernhard
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907301
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907301#60
[3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907301#75
