For some unknown reasons Mapnik has a problem with the color name DarkSlateGray, if this name occurs in a GeoJson color or fillColor attribute rendering will just fail with an exception.
Searching over the error logs I found that it is always just this one color name that makes rendering fail, but as I’m not sure about this being the only color name with a problem, I’ve now changed the Umap Json preprocessing code to do color name lookups on the python side already and replace them with their respective hex color code, so that Mapnik will never have to resolve color names by itself when processing GeoJson data files.