I was having a heck of a time trying to restore a group calendar on a client's Mac Pro server that I recently upgraded from 10.5 Server to 10.6 Server (yep, that's right— it's 2013 and I chose to use SLS instead of MLS ). It seems the Setup Assistant didn't migrate this particular type of item, so I had to go digging. One discussion here that I found had been archived, so I couldn't tag my work-around on to it:
I made a brief write-up and put it in a blog post on my site, in case anybody else (few though we may be these days!) runs across the same problem. Please have a look and let me know of any errors I might have made or if any clarification is desired.