Hi Paul, I am also getting this error, this strangely wasn't occurring a day ago, but since the model has been developed more, it now occurs and doesn't let me continue. I have downloaded the updated to 126.96.36.199.2 version but it still occurs.
Happy to send you the file if necessary.
Edit: This was with Revit 2018. I upgraded the project to Revit 2019 and the error no longer occurs.