Hi,
According to your description, I noticed that the free/busy information also couldnt be retrieved in OWA. Please confirm your Exchange version and double check whether all users cant use scheduling assistant to view free/busy information.
In Outlook client, please run Test E-mail AutoConfiguration tool to test the Autodiscover service and Availability service URL:
please open Outlook - press CTRL key - right click on the Outlook icon from right bottom corner taskbar - Test Email AutoConfiguration. Put your email address - uncheck use guessmart and secure guessmart authentication - click Test to check your Autodiscover
service.
In Results tab, please get the Availability service URL. Then access it in Internet Explorer in your local machine, and check whether a proper XML file can be returned.
Regards,