ok, then I express myself wrong. It is not a “reset” but it loads a default theme. I can say 100% that this bug was already at the initial installation, when no plug-ins were installed.
As soon as you click on the “Preview-Button” the HTML-error comes and a purple-color-Schme is loaded.
Please follow this link to see: https://www.boesemaedchen.net/wp-content/uploads/2018/08/error2.jpg
The theme is loaded in purple. And not always only now and then. If I had no backup I would have to reset the colors.