Darken function is messing with compiling

Using the standard config I always get this error when trying to compile the template/theme.

Während der Bearbeitung von Shop "Deutsch" ist ein Fehler aufgetreten: error evaluating function `darken` The first argument to darken must be a color index: 21296

Any ideas?

EDIT: I copied the current _variables/*.less files from the Github Repo just to be sure I still have the original ones. Still no comiling of the responsive theme.