Sky often not rendered in Blender 2.47. Needs close and open again.

I’m working on an animation for a while. As I have not much time, I work on it some time, leave it and come back to it some time later. This is to say that I have used the file and saved it with Blender 2.45, 2.46 and now 2.47. It may has an importance (I don’t know).
The sky button is enabled, but often when I launch a rendering after some modifications, the sky stay black instead of the light blue color setup un the world parameters. The sky is not rendered.
I switch off Blender and open again, and the sky is rendered fine.
Have encountered this kind of rendering problem ?

only when the sky vs premul button is accidently clicked, or a renderlayer option afaik.

The sky button on the renderlayer tab has to be enabled also, otherwise you end up with a black premultiplied alpha just as if you had used the Premul button. The only way closing Blender and reopening her would fix this particular oversight is if you do not save the file B4 closing.

I have already checked that. I have only one render layer, and the Sky button is enabled in the layer panel, as well as the Sky button in the render panel. It happens from time to time, but rather often. Weird.

Are you running a firewall while Blending? In my experience they cause lots of problems with Blender…especially crashes.

No, I work on an isolated PC without Internet connexion, dedicated to CG, to avoid resident softwares like Antivirus or firewalls disturbing the rendering and eating ressources.
I go on Internet on a computer which is reserved to this function, with all needed protections, and I transfer only checked and safe data or software… almost only Blender upgrades !