Apple Server 5.1.5 doesn't perform Software Update on networked machines any more (error connecting the client machines to update server)

I recently set up the Software Update via the instructions provided. All seemed OK for a while. Then I updated to Server 5.1.5 and the updates have now failed to be seen on the Server on any connected client. Updates work fine if I return the profile on the client back to previous setting- using internet backup. When using Server for updating here is the error I get. How can I troubleshoot this further? I tried setting the Server to Automatic or Manual operation without success.