Just a follow up, it also happens on the report I built whose Data Connection's data model was built using Tableau (Connecting to SQL Server, then adding multiple tables and creating the connections between them).
It's all in the name, I'm afraid. I had a similar problem with a "Completion Rate %" Workbook and reading that you had a similarly title workbook got me thinking. I eliminated my % symbol, and it worked just fine.
So get rid of your % symbol and try it out, and your error issue should be resolved.