The shadow can be deactivated by setting xfwm4 /general/shadow_dock_shadow to false for example using xconf-editor.
So this issue seems to be a xfwm problem, after all.
The shadow can be deactivated by setting xfwm4 /general/ shadow_ dock_shadow to false for example using xconf-editor.
So this issue seems to be a xfwm problem, after all.