User picker not working in "Assign Users to Project Role" page.

Clicking User Picker icon in Assign Users to Project Role page does not bring up list of users.

To reproduce the problem: Login as a JIRA administrator in JIRA 3.8. Click on the Administration tab. After the list of projects appears, click on any project. After the Project detail page appears, click on the "Project Roles: View Members" link. After the "Manager Project Role Membership for Project: ProjectName" appears, click on one of the Edit links in the "Users" column. After the "Assign Users to Project Role: ProjectRoleName" page appears, click on the user picker icon in the Add User area, on right side of page. Note that Java error message "Error on page" appears in extreme lower left of browser window, and that a window containing a list of JIRA users should appear, but it does not.