Exchange OWA stores data offline in your browser for a better user experience and performance. In older releases of Exchange, this behaviour could be turned off at the logon page by unchecking a checkbox, but since Exchange 2013, this checkbox is no longer visible on the logon page. But, this option can be turned on again by changing a configuration option of OWA in Exchange.
The option can be enabled through EMS on the CAS server. If you have more than one CAS server, this needs to be done on all of them.
Get-OwaVirtualDirectory | Set-OwaVirtualDirectory -LogonPagePublicPrivateSelectionEnabled $true
After the change, don't forget to reload IIS:
Optionally, you can also change the timeout for private and public computers through the registry: