OK I got it,
The pigment used by S_Cloud2 is a gradient y one: value increases from  0 at
y=0 to 1 at y=1 ( assuming it is not scaled) then falls to 0 at y>1
increasing again to 1 at y=2 and so on
the value describes a sawtooth patern as you can see on this image
http://users.skynet.be/bs936509/povfr/materiaux/mat-mot-geo-01.htm
so value is 1 at y <0.
It is a new behaviour of gradient in POV 3.5
try using planar instead

#declare S_Cloud2 =
sky_sphere {
    pigment {planar
        pigment_map {
            [0.00 rgb <0.847, 0.749, 0.847> ] // horizon
            [0.10 SkyBlue ]                   // horizon
            [0.20 P_Cloud4 ]
        }
    }
}


HTH

Marc

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