[fssitzung.cls] Package titlesec error

$ xelatex 180423sitzung.tex
This is XeTeX, Version 3.14159265-2.6-0.99992 (TeX Live 2015/Debian) (preloaded format=xelatex)
 restricted \write18 enabled.
entering extended mode

[...]

! Package titlesec Error: Not allowed in `easy' settings.

See the titlesec package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.121 ...     {\normalfont\huge\sffamily\bfseries}
                                                  
? H
The sectiong command you are trying to redefine
is not handled by the starred variant (eg, \part)
?