Sounds like a stray supercut. You can check by using the BSP cuts view (alt+4 I think), and if true, use the build options in the top taskbar to fiddle around with the rebuild settings. I had similar issues with DM-TMC-Narean and had to use those to solve that.
Did you do a full build, or build geometry, lights, paths, etc., separately? Did you change the optimization settings (sliders)? One problem with UED is it does not remember build settings other than the defaults. Some authors use different build settings, which can affect the map in different ways, and you would never know it. When in doubt start with a full build using the defaults, with the "build hidden actors" turned off. If that doesn't work try the other options..