@DFE Let's see if we can get more details on this issue.
Load your files again, but use a different song. Then try to save the service. If the service saves without error there may be something about the specific file you were adding.
If you still have the error, try using the File Menu to save the service as a different name. If you are still having issues, you will need to post the error logs so we can look into this more. The manual can tell you how to find the log files. I would suggest deleting the log files, starting OpenLP and going through the process to save the service and closing OpenLP. This way the log only has the information about your run with the error.