This isn't something we've had reported, so I would suspect connectivity issues or slowness between these clients and the server possibly. Can you try downloading and installing Fiddler Web Debugger (Classic) on an affected PC, go into the options and enable HTTPS Decryption, then restart it and run it while firing up Outlook and letting the problem manifest? When finished, you should see any failed connections listed in red within Fiddler. If you open a ticket and send this our way we can help analyze the results as well.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com