After moving some clients back to Exchange server from BPOS, autodiscovery in Outlook refused to find the correct autodiscover.xml file.
The registry fix below will reset autodiscovery in Outlook and make thinks work again!
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\AutoDiscover]"PreferLocalXML"=dword:1"ExcludeHttpRedirect"=dword:1"ExcludeHttpsAutodiscoverDomain"=dword:1"ExcludeHttpsRootDomain"=dword:1"ExcludeScpLookup"=dword:0"ExcludeSrvLookup"=dword:0"ExcludeSrvRecord"=dword:0More information about the issue here
