Resolution:
This is a known issue.
Following is the workaround for this issue -

- Goto “Two Columns and Grid” layout in file &ltbwfb_home>\5.2\bui\archive\WEB-INF\formrunner\templates\Layout\TwoCols WithGrid\Layout.htm.

- Delete the % preceding /PortalTemplates... :
        top.newWin = window.open( "%%RenderRequest.Attribute('ContextPath')%%" + '%/PortalTemplates/bui/pickers/Calendar/calendar.html?dField='+escape(fieldId)+'&dFormat='+escape(dateFormat), 'cal', 'dependent=yes,width=210,height=230,screenX=200, screenY=300,titlebar=yes,status=no');

- Recreate the deployment war file.

- Redeploy the application

DateCalendar Utility in Form Builderthrows HTTP 404 exception when using "Two Columns & Grid Pane" layout.