Please consider the following MWE:
\documentclass{scrartcl}
\usepackage{longtable,tabu}
\usepackage{lipsum}
\usepackage{scrpage2}
\chead[Headline 1\\ Headline 2]{Headline 1\\ 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}
I noticed that the compilation of this MWE causes the following error:
! Misplaced \cr.
\reserved@c ->\ifnum 0=`{}\fi \cr
l.17 \end{longtabu}
I can't figure out why you would want to use a tab mark
or \cr or \span just now. If something like a right brace
up above has ended a previous alignment prematurely,
you're probably due for more error messages, and you
might try typing `S' now just to see what is salvageable.
For one document, I needed a two-line page header and got this error, which — I guess — belongs to the tabu package. I can't find any error in it and would be delighted about help how I can get a line break in the page header without this error.