If you are using Linux with XFce4 desktop environment and you've got either a "No running instance of xfce4-panel has been found" or "Modifying the panel is not allowed" error message then probably you started digging on Internet for a solution and finally you've found this page. Isn't it?
Now, I assume you've found many of these web forums that teach you to remove some files or folders, sessions, caches, whatever. You did all of this but nothing helps? Well, probably you did it but not in the right order.
First of all I assume that your ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml file is all right. By this I mean it is not damaged/corrupted whatsoever. If that's true then probably you get all these errors because somehow its cached version has problems. Or even worst, the previous session got problems (like home partition hasn't been unmounted properly, whatever).
First of all let's understand a bit how the panel is stored and how is working. The xfce4-panel is just a simple application that allows you to define one or more panels and for each of them one or more items (shortcuts/launchers). The panel definition is stored in the file I mentioned before (xfce4-panel.xml). Each panel's item is stored on a different INI file and you basically can find them at ~/.config/xfce4/panel/*. Each of these files have a funny name like 14296861956.desktop. The big number looks like a unique timestamp to me where the last digit is there just to make sure the file got a really unique funny name. Anyway, DO NOT TOUCH ANY OF THESE. They are the panel's definitions. You delete them, you have to recreate/redefine them again together with whatever items/launchers they had. If you do this then you better have a backup first!
Now, back to the topic, the solution to the above error will be to clear the cache and remove the session garbage. But what's more important than that is to LOG OFF from the active desktop session and go back to the basics (the terminal):
- log off your desktop session and make sure you stay so (otherwise whatever we fix here it will be overwritten by the active desktop session at the next log off)
- switch to a terminal mode (Ctrl+Alt+F1 ... Ctrl+Alt+Fx, x=2..11)
- if necessary log to the terminal with the same user/password as you use for the desktop session
- while on terminal delete whatever exists saved on the cached session:
rm -r .cache/sessions/*
- finally log-in back to your desktop environment and hopefully the message gone
Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.