In tcolorbox-tutorial-poster.tex, an undefined control sequence \interlineskip is found, see before skip=-\interlineskip on line 1375.

There is a format macro called \nointerlineskip, but never \interlineskip, not even in packages. Sometimes people mis-spelled \nointerlineskip without the leading "no", see for example a pgf commit pgf-tikz/pgf@5466590 in 2010 and a comp.text.tex discussion in 2006, but setting before skip=-\nointerlineskip is still wrong.

Actually the setting before skip=-\interlineskip given to \tcbinputlisting never took effect, because this \tcbinputlisting is the first material inside \posterbox, so no before skip would be inserted. This explains why it caused no errors since committed in 8a2607f (4.10, 2017-07-05).

Thanks to TeXstudio's code highlighting and @mbertucci47's excellent contributions to cwl (completion word list) files.