Theses, Books, Title pages ⇒ Missing Headline separator in Master / Doctoral Thesis

[mgo4107]

After using the following code in

%Masters/Doctoral Thesis
% LaTeX Template
% Version 2.5 (27/8/17)

\documentclass{book}
 
\begin{document}
\chapter{Heading on the page}
Text
 
\begingroup
% no newpage
\let\cleardoublepage\relax
%\renewcommand{\cleardoublepage}{}
\chapter{Heading one the same page}
Text
\endgroup
 
\chapter{Heading after at least one page break}
Text
\end{document}

to prevent pagebreak between chapters the headline separator disappeared which was
activated via

%headsepline, % Uncomment to get a line under the header

Can somebody help to switch it on again?
thanks.