jqgrid.jqGrid('jqGridExport',{exptype:"jsonstring"})

in some part the json string is malformed, just right here:

.....     "sord":"asc",    "filters":"    },    "userData":{},    "treeGrid"  ....

the filters attribute has only one quote.

Solution (hardcoded):

var badjson = jqgrid.jqGrid('jqGridExport',{exptype:"jsonstring"});

var goodjson = badjson.split('"filters":"    }').join('"filters":""    }');

i hope this get fixed in new releases. the one with the error is: 4.1.2, and i think that in the 3.8 it works fine.