Robin Fairbairns has released the version 2.16a (2009/03/28) of the rotating package, there he has corrected a bug introduced in v2.16 (2009/03/16). Using 2.16 you could get the error message ! A <box> was supposed to be here.
<to be read again>
\ignorespaces when using a rotating environment like rotate or sideways.
The bug and it’s cause were discussed by Jürgen Spitzmüller and Ulrike Fischer (”…rotating uses \color@hbox in a context where a box command is needed…”) on comp.text.tex and Robin fixed it by adding

\AtBeginDocument{%
  \providecommand\color@hbox{\hbox\bgroup}%
  \providecommand\color@vbox{\vbox\bgroup}%
  \providecommand\color@endbox{\egroup}%
}

to 2.16.

If you encounter this error just update the rotating package, perhaps by a package manager like mpm or tlmgr. Thanks to Jürgen, Ulrike and Robin for the quick solution.

This topic was also discussed on mrunix.de.