the most common reason for this is that OAB is requiring HTTPS connection, and outlook using not using HTTP connection, How to know ?
Hold the CTRL key and Right Click on Microsoft Outlook tray icon and select Test E-mail Autoconfiguration.
Notice that OAB URL is using HTTP. so now lets check it on the server side
Login to Exchange server that had Client Access Server
- Open IIS --> Default WebSite --> and click on OAB virtual directory
on the Feature View panel click on SSL Settings and make sure that Require SSL is deselected
You have to do this on all CAS server you have.
Thats it, enjoy your day.
These errors can also caused by bad autodiscover configuration or SSL problem, alot of reason, but this one I write is the common