This morning 13-Sep, Webmail login works OK. Selecting the "Start Page" by clicking the toplevel folder gave an empty page (status bar showed loading not finished). The folder tree showed OK. Clicking the Inbox worked fine. this repeated on going back to top folder.
The problem has gone away now. But it did happen. Iogged a ticket because it was unusual that no page would come back. However, CS could not see a fault.