Yep, my fault !
In the installation directory, file app_settings/shaders/ee/class1/deferred/skyV.glsl, line 142, replace "additiveColorBelowCloud" with "add_below_cloud" to read:
Alternatively, use the Windlight renderer (i.e. un-check "Extended environment shaders" in the "Preferences" floater, "Graphics" tab, "Renderer settings" sub-tab) ...