Hi Rikard,
It appears that I could solve the 404 redirection of the start page. I realized that I need to define the start page in two different areas:
(Design => Customizer => Startseiten Einstellungen (starting page settings) and also under Enfold Theme Optionen => Startseiten Einstellungen (starting page settings). While I do not know when or how this changed, I could now fix it, and hope it will remain so. So, I would rather not update the theme for the time beeing on this site, but only use the 5.4 on another site, which I currently develop to avoid any risk of another problem. Still, would it be possible to go back to the older 4.8 version by again overwriting a 5.4 installation?
thank you for your feedback and help anyway.
kind regards
Rudi