From the error message, I'm guessing that you're using multiple data sources (maybe just 2). Create an extract for both data sources and see if that helps/fixes it.
I also don't think COUNTD will work, but it depends on your data.
Can you show some of your data - make sure it's dummy data, but if your volunteers are Santa Claus and Mrs Claus then I won't question it at all
I think it's because of the Data Blending and COUNTD() operation. This thread has some excellent information that Jonathan Drummey has provided and should give you some direction.
Some additional links,
Also, mock up some data and attach packaged workbook for the users to troubleshoot.