Viewport, Blender Internal, all materials going black

Hello, Blenderers, have I got an issue.

For no apparent reason all materials in my (fairly complex) scene ceased to get lit in the Material mode, and are plain black (except those few set to be shadeless). However, once I select an object, it becomes lit and retains some screen-space AO, but otherwise no material settings are showing - no textures, etc.

I have a Sun lamp and Environment lighting on, but it still doesn’t help.

In another file that contains some part of the first scene everything works fine.

What may cause such things?