Every time I start Xfce 4 the panels are missing and a window with that notification appears. Then I press the button 'Execute' on the window and the panels reappear. Possible cause? Xfce 4 was the only DE I chose during installation. The Debian version is 9.

Possible cause: XFCE may not have installed properly during installation. (stuff like that has happened to me)

On this page is the e-mail address that you can write to the Debian XFCE Maintainers. I'd start by writing to them.
https://bugs.debian.org/cgi-bin/bugr...cgi?bug=882792

In the meantime maybe install another DE of your choice.
Remove the XFCE DE and than try re-installing it:-

Ensure that you have the current version of XFCE:-

I think it's 4.12 now and looks like 4.14 is still in it's development cycle.
https://xfce.org/
https://wiki.xfce.org/releng/4.14/roadmap

Other XFCE Bugs:
https://www.google.com/search?client....0.EFTxYpyzcMs

Maybe look up some debugging tools to find out why the panels are missing.
https://www.thegeekstuff.com/2011/12...itoring-tools/

HTH