I am uploading project files right now ( 5 gigabytes )
I had another crash to desktop a moment ago 
Here are the details: SpeedyShare
log file SpeedyShare
dump file SpeedyShare
Hey! 5 gb is a lot, I don’t think I’ll be able to download it in a reasonable time (poor internet). Do you have 4+ gb of textures? Could you delete those as well since I don’t think textures are going to matter a lot while building lighting. I just need to see what kind of meshes you’ve placed in the scene and if you’re using instancing etc.
Hi wielkiczarnyafgan,
Apologies for the delay, There have been a lot of posts with Lightmass and how to use it properly lately. While the crash shouldn’t be happening, to which I’ve entered a bug report for with ticket UE-12184.
Ideally though, Xenlock is right that there is a lot going on with space usage and lightmass. It’s a very intensive process for lightmass to build lighting and can use all RAM to do so, then when it runs out of RAM will use virtual cache and write to the hard drive, which is a slow process and can also account for the 10+ hours to build lighting.
You also have to take into account the number of meshes that are being calculated, their lightmap resolutions, and the number of lights casting shadows that are affecting each mesh.
Looking at the log you have ~11k textures that are just for the Lightmap and Shadowmap textures in memory. This also totals to over 1.5gb of textures. I don’t recall reading above that you had a large world open level, but from this it would appear possible. Typically with these types of worlds you would want to use Dynamic lighting in place of static lighting due to the memory footprint that lightmaps and shadowsmaps can take.
I’ve entered the crash to be investigated, so hopefully that can be resolved when an engineer takes a look.
The error is because of the same reason. You’re overloading lightmass. Check that you don’t have:
(a) huge meshes or (b) lots of clustered meshes or
(c) high lightmass resolution or
(d) no lightmass importance volume
Try reducing the number of objects and see the lighting build log. You’ll see the meshes that are taking the most time to build ordered sequentially by time. Add a few meshes each time you get a successful build and you’ll eventually figure out what’s causing the problem.
A ) I have several landscape meshes, but they already had their lightning built
B) I don’t see any clustered meshes on my map
C) This can be issue - how can I change the lightmass resolution - please tell
D) I have one lightmass importance volume, that covers entire map
To lower Lightmap resolution, select the static mesh and in the details panel, reduce the ‘Lightmap Resolution’ until you reach a stable build and comfortable trade-off. If you’re using BSP though, increasing the Lightmap Resolution will actually reduce the quality.
Hi Guys! I have decided to go for the Dynamic lightning - no issues so far ! 
Hi guys,
I think I have found the problem.
@wielkiczarnyafgan, have you used sources and could you verify it for us?
Thanks,
Jarek
I was asking if you got your engine from GitHub and built it yourself or got precompiled version from UnrealLauncher?
If the former, I can instruct you how to fix it, then you can build it and check if it works now. Otherwise you will have to wait for another release (or hot-fix).
Hi Jarek. What kind of “sources” do you mean ?
Jarek - sorry for late reply I have got the precompiled version from UnrealLauncher
Alright, so are you willing to download sources from GitHub and try if my solution solves your problem?
Hi ! I could try.
Regards
Alright, so if you sync the code then open Engine\Source\Runtime\Core\Private\Serialization\Archive.cpp file. Then find int32 EndPosition = Tell(); line and change the type to int64. Then of course build the engine.
Should fix your problem, but please – let me know!
Thanks,
Jarek