Just debugged your page.
Seems your host append some content to generated response.
For example, the navigation request (double-click) get javascript block in response:
"var folderTree = window['WFM_' + context.ClientID + '_FolderTree'];
window['WFM_' + context.ClientID + '_SelectedFolderPath'] = ['facf09a4','62e52a17']
if(folderTree) {folderTree.Navigate (window['WFM_' + context.ClientID + '_SelectedFolderPath'],0);}
var dir = '%2fContent%20Folder%2fmedia'
WebForm_GetElementById(context.ClientID+'_Directory').value = dir;
var address = WebForm_GetElementById(context.ClientID+'_Address');
if(address) address.value = decodeURIComponent(dir);
var searchTerm = WebForm_GetElementById(context.ClientID+'_SearchTerm');
if(searchTerm) searchTerm.value = '';
window['WFM_' + context.ClientID].ClearSearchBox();
WFM_FileManager1_Controller.OnRefresh(context,null)
<!--SCRIPT GENERATED BY SERVER! PLEASE REMOVE-->
</textarea></xml></script></noframes></noscript></object></layer></style></title></applet>
<!--SCRIPT GENERATED BY SERVER! PLEASE REMOVE-->"
Where <!-- SCRIPT .... --> block is not originally what IZWebFileManager produces.
You should remove it, possible it is configurable with your host service.