Too many lights
There are more than 8912 lights set to have influence over dynamic MDL lighting (MAX_MAP_WORLDLIGHTS). This is usually caused by having too many texlights at too low of a luxel scale.
Tip:This is a compiler limit, not an engine one, and as such, a modified VRAD can bypass it, such as 
Mapbase's. Nonetheless, having too many lights in the worldlights lump will have negative consequences to performance, so a custom compiler wishing to bypass this limitation should provide a method to have lights that affect baked lighting, but aren't added to the worldlights lump.