I would guess it has something to do with layergroups. Have a look at this recent discussion lesson7-CAM: mismatch between # of lrs and of layer groups.
To be more specific, one would need to pass an array of learning rates equal to the number of layer groups. So first check the number of layergroups, and for each layergroup there should be a learning rate.
Hope that helps. I will perhaps try to add this to the notebook as well in some days.