If there’s ever trouble with iChat server and ServerAdmin doesn’t let you do anything, you can fix things up based on this Apple Support article:

This can occur if the /Library/Preferences/ichatserver.plist file has been deleted or is problematic. If the com.apple.ichatserver.plist file is deleted a new default version of this plist will be created.

Refresh the Server Admin view for the iChat service. The Start button should now appear and the service should be configurable.

You can reset the iChat database with (you should make sure you have a backup!):