Any ideas why this "light leak" is happening? I think I've checked everything but couldn't find a solution to fix that... The blender file is attached as well if you want to have a look. Cheers!
Afaik, implicit problem with AC engine. Even Kunos tracks suffer from this (e.g. have a look at the pit buildings at Silverstone). Sometimes you can cheat with an additional plane/box behind the object and which could block the light, but not always.
car models have the same issue. Normally doing a very simple bevel to eliminate the 90° angles is enough to stop it
I'd also suggest running less extreme shader settings, so ambient parts aren't so dark and areas in diffuse aren't so bright. The leakage will be less obvious then.