For second level grouping, the same fix for Issue 31 needs to be applied for sGroupData2.

When using second level grouping, you get a null reference in toLowerCase() in the _fnGetCleanedGroup b/c the aData is a map now instead of an indexed array. You must use the fnGetData() call to translate the column index to the actual data value:

Line: 423 should be:

sGroupData2 = this.fnGetData(nTrs[i], properties.iGroupingColumnIndex2);

And comment existing lines 423-425 to fix the problem.