UE4Editor.exe crashes when I try to build lights

Every time I try to build lights (both for templates and custom projects) the editor crashes.

Everything works fine, building geometry works as it should, but for some reason each time I try to build lights the Editor crashes immediately.

This problem first appeared after I had to format my C drive and re-install a clean Win7.
I was using UE4 on this same hardware with the same system version before and did not have any problems with the engine running.

Any ideas as to how to fix this?

After I updated .net to 4.5 the Editor no longer crashes. I do get a “Failed to connect to Swarm.” error and the shadows are not built anyway.

Any ideas?

The engine just started building lights again. I’ll have to confirm this in a bit, but somehow the issue seems to be gone.