404 error when reuse refreshpart in hudson-behavior.js

[James Yin]

I am trying to enhance the backup plugin, and it will show a progress bar 
in side pane when a backup task is running. I think reuse refreshpart 
function is good idea.

but I get follow error when the progress bar auto refreshed:
POST http://localhost:8080/backup/backupTask 404 (Not Found) 

Is anybody can help me?

Hudson version is 1.375

the jelly files as follow:
src/main/resources/org/jvnet/hudson/plugins/backup/BackupWidget/index.jelly
<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" 
xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" 
xmlns:f="/lib/form" xmlns:i="jelly:fmt">
    <st:include page="backupTask.jelly"/>
</j:jelly>

src/main/resources/org/jvnet/hudson/plugins/backup/BackupWidget/backupTask.jelly
<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" 
xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" 
xmlns:f="/lib/form" xmlns:i="jelly:fmt">
    <l:pane width="3" title="${%wiget.title}" id="backupTask">
       ...    
    </l:pane>
    <j:if test="${ajax==null and !h.isAutoRefresh(request) and 
h.hasPermission(app.READ)}">
        <script defer="defer">
        refreshPart('backupTask',"${rootURL}/backup/backupTask");
        </script>
    </j:if>
</j:jelly>