I found this thread searching for the same answers when I figured out just now that if you want a large we'll say as in my case .. terrain like 5000, 5000, start with the default size of your mesh or a smaller version and incrementally add more to it. It seems to process more easily due to only having to process the added content.

Light transform was where it was happening to me also and this will stop you from testing .exe of app because .exe process tries to copy a file that is still being used by another process. .... The stuck light transform job (10 for me) that appears to just do nothing as someone here stated, still has a handle on the file apparently and it will keep you from creating a .exe file of your app. Hope this helps someone. I have done just what I am saying and now my map is almost the size I want with no issues. All jobs completing fine.

Unity does not like to work on large jobs apparently... ;)

NIGHTCODER99 have 100% right, it seems that Enlighten precompute (clustering) has a problem with big meshes, "BIG" means not in terms of vertices or faces but their scale.

Steps to reproduce:
You can freeze GI simply putting one plane on the scene (even Plane object from Unity) and scaling it 16 times (16,16,16). Now try to build GI lighting (even without baking textures - only realtime), job freezes at clustering.

Workaround:
Instead of using one plane 16x16 try to create more planes but smaller in dimensions, the problems solves immediately. For example 16 planes scaled twice time (2x2).

just check the meshes you are using. for me i was using a mesh for my floor and i was scalling it to 20 units . i deleted and duplicate it 20 times and used it worked.

it turned out not specifically nonmanifold geometries cause the problem
I have a closet full of shoes I found one of shoes which hasn't much poly is the problem
it is defiantly related to bad geometries

after weeks of Trial and error I found the problem
the problem is related to nonmanifold geometries

to fix bad geometry in maya go to mesh / clean up / check nonmanifold geometry

Please let me know if you managed to solve your scene problem

I'm seeing this on 5.0.1f1.

Yeah, when I start Unity now, my sun suddenly always light looks orange and has hard shadows. Before when I started unity, this happened briefly then stopped doing that. At the bottom it says the same thing: Clustering jobs 5/11. Forever. This happened on 5.0.0.f4, and I'm upgrading now to 5.0.1f1, so we'll see.

Same Error

This is happening to me at Light transform ; unity pro 5.0.1f1 - never moves beyond this.
I am baking the scene to get past the other issue of clipping and flashing on shadows...

I have a very simple scene where unity locks up my computer, does nothing then crashes.. Lighting really needs to get fixed.