Hello Jürgen, > So... what am I still doing wrong?
The issue is of course with the diacritics. The text used in the index entry is used in command definition used for cross-referencing. Because you use TeX commands for diacritics (like \"o), it produces an error. It works with direct Unicode characters (ö), so easiest solution is to use Unicode. And Unicode engine, preferably LuaTeX: tex4ebook -lm index -c myconfig.cfg -e mybuild.mk4 Test2.tex should work. Best regards, Michal
