longtabu and scrpage2: line breaks in page header cause compilation errors

the page head is catching longtables version of \\ easiest is just to avoid the problem by using the alternative form

\chead[Headline 1\linebreak Headline 2]{Headline 1\linebreak Headline 2}

longtabu redefines (like every tabular) \\ and in the header this definition is still active and leads to chaos. Use another command:

\documentclass{scrartcl}
\usepackage{longtable,tabu}
\usepackage{lipsum}
\usepackage{scrpage2}
\let\ORInewline\\
\chead[Headline 1\ORInewline Headline 2]{Headline 1\ORInewline Headline 2}
\pagestyle{scrheadings}

\begin{document}
\lipsum[1-5]
\begin{longtabu}{XXX}
    Col1 & Col2 & Col3 \\
    Col1 & Col2 & Col3 \\
    Col1 & Col2 & Col3 \\
    Col1 & Col2 & Col3 \\
    Col1 & Col2 & Col3 \\
    Col1 & Col2 & Col3 \\
\end{longtabu}
\end{document}