On startup I get the message "no running instance of xfce4-panel found" multiple times though xfce4-panel starts after the first click on "ok-start-it". Though the problem appeared after an update to xfdesktop4 4.12.3-3 (Debian stretch packages) it may still be a configuration issue. Where can I find the settings responsible for the invisible autostart of the xfce4-panel?
The panel is started by xfce4-session, and the config file can be found here: ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-session.xml Clearing the session cache may resolve the described issue (Xfce Settings > Session and Startup > Session).
@Elmar: As Theo mentioned the location of the file you were looking for and your bug is gone I would consider this bug closed.