Dear wintpe,
I have been thinking about your suggestion, but I do not believe that removing programs influences user settings. The choice appears during login, so the setting must be in the user directory. I removed all XFCE settings via a system on an external HDD and on rebooting and logging in I found that the choice had disappeared. So the setting is in the home directory. XFCE has a window with all settings, so I had a close look at all of them hoping to find the one that had escaped me. Finally I found:
https://www.dropbox.com/s/t3ux18ixxchkp ... 7.png?dl=0Somehow the encircled option must have been ticked.
Thank you, Doktor5000, for your help. I am proud to say that I found your solution all by myself today.
Greetings and thanks both of you for your everpresent help.
Erik Jan