In my blend project I get the following error:

'StaticResourceExtension' is not valid for Setter.Value. The only supported MarkupExtension types are DynamicResourceExtension and BindingBase or derived types.

This causes previews in the blend project to fail, but in Unity there's no errors and it works fine. Any help?

It could be a problem with the order of the definition of the resources. A StaticResource can only be used if the referenced resource was defined before in the same dictionary or in a merged dictionary.

Can you check that?