Hope this may save someone hours of work and frustration. The problem involved Blender, Make Human Model and Chicken, and they all worked fine. When the animated model was exported via chicken the head had what looked like a 2 x 4 stuck in the back. Some of the pixels were stuck in space and had created a long trail.
Turns out the internal mouth structure of the model was at fault. The tongue was not weight painted to any bone. How to get at the internal mesh with the head in the way? The solution was rather easy: hide the surrounding mesh. Zoom into the area, select with the B key for several points or faces, H for hide.
Rotate the model til the entire head is hidden. Then delete the internal parts, teeth and tongue. Or you may try to weight paint the parts to a bone, that may also work.