Rectangular static lighting glitches

Hello.
I’m trying to make a nice forest level, but getting weird issues:

  1. Rectangular light/dark areas on the landscape. Small rectangles, large sections etc.
  2. Some trees’trunks get black.
  3. Rarely small rainbow areas appear on the ground or objects, disappear when I hit “play”

The lighting settings are standard. I’ve tried to replace the trees by the completely different ones - still the same issue.
I’ve tried many things:

  1. Rebuilding many times in a row gives the same result, rectangles on the same place.
  2. Moving trees changes the rectangles.
  3. Replacing the landscape material by a simplest color-only one changes nothing.

Tried different settings for lightmap scale etc - nothing makes much difference. I’m running out of ideas. What can be wrong?

UPD: looks like it happens when a lot of complex meshes (trees) are pretty close to each other. I’ve tried 4 different tree types from different packs. Also I can reproduce it on two different computers.