JavaScript error on IE when selecting reporter via sub-task's quick create form

Our customer has configured jira-application.properties file in such a way that also the reporter field appears in the sub-task's quick create form.

When using IE6 or IE7, the user picker pop-up shown "Error on page" message. The more detailed error message is:

"Line: 284
Char: 5
Error: 'elements.reporter' is null or not an object
Code: 0
URL: http://.../secure/popups/UserPckerBrowser.jspa?formName=jiraform&element=reporter

With Firefox selecting reporter works fine.