svmult doesn't play well with babel

MWE using svmult class from https://resource-cms.springernature.com/springer-cms/rest/v1/content/20568/data/v8

documentclass[graybox]{svmult}
usepackage[british]{babel}

begin{document}

section{Introduction}
label{introduction}

Foo Bar

bibliographystyle{spbasic}
end{document}

When compiled via pdflatexmk gives me the following error:

./authorsample.aux:5: LaTeX Error: Missing begin{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.5 newlabel{introduction}{{1}{1}}[
                                    None]

authorsample.aux is

relax 
bibstyle{spbasic}
immediatecloseoutminitoc
@writefile{toc}{contentsline {section}{numberline {1}Introduction}{1}protected@file@percent }
newlabel{introduction}{{1}{1}}[None]

How can I use svmult with babel?

diff --git a/svmult.cls b/svmult.cls
index 2c39e97..62287b8 100644
--- a/svmult.cls  
+++ b/svmult.cls  
@@ -2460,13 +2460,13 @@ toz@{[email protected]}box0}}}}
 
                         %%%SOPHIE TEMPLATE END%%%
 
-usepackage[thmmarks,thref]{ntheorem} 
-theoremstyle{nonumberplain}
-theoremheaderfont{bfseriesitshape}
-theorembodyfont{upshape}
-theoremsymbol{ensuremath{square}}
-newtheorem{proof}{Proof} 
-gdefNoneSymbol{}
+%usepackage[thmmarks,thref]{ntheorem}
+%theoremstyle{nonumberplain}
+%theoremheaderfont{bfseriesitshape}
+%theorembodyfont{upshape}
+%theoremsymbol{ensuremath{square}}
+%newtheorem{proof}{Proof} 
+%gdefNoneSymbol{}
 
 endinput
 %end of file svmult.cls