jQuery Grid Plugin - jqGrid

Hi to all!, i have a problem when i use grouping function, groups are correct, but when i do click in '+' or '-' , (depends how i configure this initially)i have an error. 

In IE8 sends this error:

Mensaje: Object doesn't support this property or method

Line: 387

Caracter: 120

Code: 0

URL: http://www.test.com/jqGrid/js/.....s?v ( the URL is an example)

and with Firefox 3.6.9 with FireBug:

f("#" + b).nextUntil is not a function

http://www.test.com/jqGrid/js/......min.js?vX

Line 387

This is my grid:

jQuery("#list10").jqGrid({
   url: buildUurl,
 datatype: "json",
    colNames:['Id Propietario','Propietario','Nombre Propietario', 'Apellido Propietario'],
    colModel:[
     {name:'id',index:'id', width:55, hidden:true},
     {name:'title',index:'title', width:140},  
  {name:'nombre',index:'nombre', width:140},
     {name:'apellido',index:'apellido', width:140},  
    ],
    rowNum:10,
    rowList:[10,20,30],
    pager: '#pager10',
    sortname: 'id',
 loadonce:true,
    viewrecords: true,
    sortorder: "desc",
 multiselect: false,
 ignoreCase: true,
 caption: "Propietarios",
 grouping: true,
 groupingView :
    { groupField : ['title'],
     groupColumnShow : [true],
     groupText : ['<b>{0}</b>'],
     groupCollapse : false,
     groupOrder: ['asc'],
     groupSummary : [true] } ,

});

  jQuery("#list10").jqGrid('navGrid','#pgwidth',{edit:false,add:false,del:false});

I want to know if i´m doing something bad or if is there a bug.

Thanks in advance,

Nicolás.