To learn more about the problem, try turning on Exchange tracing. Open the EMC and go to Organisation Configuration > Client Access > Outlook Web App to allow Exchange tracing. Click Edit after choosing the OWA virtual directory you want to enable tracing for. Select the Enable tracing check box under Logging.
Once tracing has been enabled, you can duplicate the problem and gather the trace logs. You can get the trace logs by launching the EMC and going to Monitoring > Mailbox > Activity Logs. To search, select the Trace log type. The results window will display the trace logs.