Do you have multiple streams displayed on the same page? It's possible their update functionality is conflicting somehow. If so try disabling any other streams on the page then see if the group stream updates fine. The easiest way to test this is simply open 2 browser tabs for the same group and post in 1 browser tab then wait for the other browser tab to update; this test is working fine for me, but I don't have multiple streams on the same page at this time.