Try switching off the mask (set the mask’s Blending Mode to None) to see if the problem persists. Fix the mask if this step solves the problem. Otherwise, ensure that the background layer is not intersecting other layers. You can more easily view intersecting layers in a view other than the Active Camera View.
HTH
RoRK
Intensive mocha & AE Training in Singapore and Other Dangerous Locations
Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects CS6 ACE