I just did this. here's how I did it:
I can confirm that this worked to reset only the Calendar service on Server 5 be prepared to perform step 2 by setting the permissions or by having a terminal window already navigated to that folder:
- Toss the Server.app to the trash. Wait for the script to trigger the pop up confirmation. Click OK.
- Empty the contents of /Library/Server/Calendar and Contacts. You can either do this on the command line with 'sudo rm -R' from inside the folder, or, if you aren't comfortable with command line foo, then add yourself as a user to the folder using the Finder's "Get Info". click the wheel and apply that to the
- Move Server.app back to Applications folder.
- Start Server. Wait for the migration scripts to run.
- Start services.
Calendar, wiki, contacts... all of them started nicely.
You may be able to accomplish this without trashing the server.app by emptying the folder, then deleting the /var/run/.ServerMigrationDone file... I've never tried that method, but perhaps next time I will. I do know that the steps above reset calendar to "out of box". My other services were left intact.