I updated to version 5.28.0 in the meantime, but I'm not able any way to modify the Keyboard shortcuts in the editor preferences again ...
Sorry for my stupid questions, I don't know if I'm blind or so ...
@Kovid: I read the section about the snippets, but as far as I understood, I have to type in a trigger, e.g. "<a" and press Ctrl+J" ... but if I mark my text and type in a trigger, my text will be overwritten .. it should work like the shortcut "Ctrl+B" which changes "text" (which I highlighted or selected before) directly into "<b>text</b>" without typing any trigger at all. But unfortunately I'm unable to figure out how the shortcuts can be customized ...