OK, that was the problem:

At the beginning of the race my cars stood on pure terrain, not taking any lighting information from it. My terrain has no shadow map because Im still using A7 EXTRA.
So the cars appeared in their normal material settings, given in MED. Then they started racing. The first time they got in contact, or were over a shadow map of a map entity, the took the lighting/shadow values of that entity. When leaving the entity they kept these values until they reached another entity, again taking the light and shadow infrmation from the entity.

How I solved it:

I placed a shaded block-entity beneath the terrain where my cars are at game startup and also dimmed the material settings for them down a bit. So the cars have a lighting
sufficient for 95% of the race track, consisting of terrain onyl. Other lighting probs will be solved ba putting a map entity benath the terrain, when necessary.

NeoDumont

Yes, I think about upgrading to A8...