After the installation of Service Pack 2 on Exchange Server 2007, all Mac Outlook 2011 clients can't connect to the server. All other clients (Windows Outlook 2007, iPhone, ...) don't have problems connecting.
The Mac Outlook 2011 clients are manually configured with the server's adress using SSL access. The connection's certificate is self-signed. That was never a problem though. The Macs haven't joined the AD domain.
I enabled logging on the client. It only shows some issues with Exchange's Autodiscovery feature which isn't configured on the server. It seems that Outlook 2011 doesn't even try to connect. At least there is no sign in the error log.
There are no hints in the server's event log either.