TeX Live 2018 broke compilation of labbook-based document

Your class is 8-bit encoded and uses non-ascii chars in the code. this breaks as the default encoding of latex is now utf8. You can avoid the error by using \UseRawInputEncoding:

\UseRawInputEncoding %without it you get the error.
\documentclass[12pt]{labbook}

\begin{document}
\experiment{NJOY}
\end{document}

The labbook.cls uses § as an internal delimiter for a macro argument.

If I add

\chardef\labbook@us=\catcode`_
\catcode`_=3 % funny catcode

just after the line \RequirePackage{makeidx} \makeindex and substitute all § characters with _ (which is otherwise not used in the class), doing also

\catcode`_=\labbook@us

just before the final \endinput, then the example file compiles without any problem.

Using an 8-bit character for that purpose is wrong anyhow; however, you can add

\UseRawInputEncoding

at the start of any file using labbook.cls and the compilation will go smooth.