I was having an issue where I would get an error while trying to drilldown on certain chart series. I discovered that this was only happening on series where the department name (X axis) had a ampersand (&) in the name. I simply remeved the ampersand form the department name in the list and all works well now. My speculation is that the ampersand causes an issue with the underlying query that is passed back to retrieve the drilldown data. Hope this helps someone out there having the same issue.