I had the same problem for awhile and I finally solved it by taking out
some extraneous links from my URDF that were simply adding some
cosmetics (e.g. some bracket mounts hanging off one of the servos). I'm
not sure if this causes a branch in the kinematic tree that messes up
MoveIt or something else. There were no errors when I ran xacro on the
"bad" URDF.
--patrick