@JohnM Thanks for your explanation - not that I really understand it :-)
Sorry it's taken me so long to reply, but can you please the following behaviour.
In my original post I explained how I sometimes use a service created on a different laptop which leads to the error I mentioned. However, I've found that I sometimes get the same error when using a service that I created on my laptop. I open last weeks service, and Save As to create this weeks. I periodically successfully save the new service as I add new songs and remove old ones. But eventually I get the error message I mentioned above. How is it able to successfully save the file numerous times before failing? If this behaviour is expected, is it bad practice to use Save As and instead should I create a fresh service every time?
Thanks for your assistance.