I was encountering a couple of issues with my OS X Server installation.
First, I found that ssh logins to my server, which I access in the dot-local notation (i.e. server-name.local) were suddenly very, very slow. Using ssh without Rendezvous/Bonjour was still normal.
Second, iCal suddenly stopped updating its data to the local WebDAV server, which I also access using the dot-local notation
After some troubleshooting, I tracked down the cause of the problem. The problem was that I had enabled the new OS X Tiger Server 'Stealth Mode for TCP' by checking the box in the Firewall section of Server Admin. Removing the checkmark and saving solved the problem.
Mac OS X Hints
http://hints.macworld.com/article.php?story=200603010930329