Join the club mate, this has happened to before in the past. I've even tried adding a normal light entity near it, all it did was make the lighting look shit. I tried light scale, placing hints (thinking it might have been caused by vis leafs) and still nothing. Only way I got around it was to delete the light_spot and place a normal entity instead. Of course is didn't give the effect I wanted, but it stopped that black fugly shit.
A few weeks ago on the Egyptian map I'm making, I had a room in the similar shape you've got on the last screenshot, and even with a normal light entity I got the black curse lol. But this time I saved it with hints. That corner you've got on a angle, make sure to place hints across, so that the compiler thinks it's square.