I also had a look at LO help . It says about Hidden Option: "Hides the
selected characters. To display the hidden text, ensure that Formatting Marks
is selected in the View menu. You can also choose Tools - Options - LibreOffice
Writer - Formatting Aids and select Hidden characters." But I couldn't make it
visible with Formating Marks or setings in Options. So I'm not sure, if this is
another bug (and to solve this bug would also solve the prolem that is
described here) or is LO Help wrong.