Ok, here we go.
The 2.6 renderings settings dialog here set /sim/rendering/shaders/skydome for skydome scattering.
My preferences.xml has as first line after the <Propertylist> <local-weather include="Environment/local-weather-defaults.xml"/>. That's it.
Setting /environment/terminator-relative-position-m manually does have an effect. Setting it e.g. to 3M creates a nice noon sky as you said.
It seems, there might me some bugs in the 2.6 release, like the missing integration or the local weather rules?