The easiest thing to do is reinstall Lion Server.
Go to-
System/Library/Core Services/ serverversion.plist
move serverversion.plist to trash but keep to put back later
Now move the server app from applications folder to trash
Download Lion Server from the app store and run an install
You will find services are configured afresh but it may not launch so just cancel at the end
put back the serverversion.plist from the trash
Open the applications window then launch Timemachine and restore the Lion server app.
I used the Lion server app from last known good confuuration when it was working well.
This procedure above works for me as a shortcut reinstall of lion server.
Instead of using Timemachine you could try putting back the server app from trash instead. I have not tried that as I wanted the app itself from last known good confuration where it was working well in case the app itself has problems.
The problems with Lion server in my experience that can occur are usually ones related to configuration of services it provides.