As told, for me it works without problems. My example had 2700 tris and 91 bones.

Another thought, Brad releases new versions every now and then. And sometimes this new versions needs newer versions for the im- and exporter plugins too. I needed for example to redownload FBX plugin to show FBX in the dialogues again when i installed current UU3D V3.16. Maybe you have such an outdated plugin version that doesn't longer work proper with your UU3D version. Worth a look