I have found that in Maya 2013 there is a bug when you trying to switch between “Load By” items in the dialog box Save Weights.
Problem is in the 663 line of the main script cMusclesSaveWeights.mel
It was found that “Point Position Match Tolerance” is visual element floatField. But in 663 line script code tries to change value using “textField” element type, possibly from a previous version of the script. This causes an error.