I enabled remote errors to pinpoint the problem.
I identified that a column in a particular dataset (one of my views) was throwing an error.
So using a tool “SQL Delta”, I compared the development version of the database with the live version on the reporting server. I noticed that one of the views had an extra column on the development server, that was not on the live version of the db.
SQL Delta generated the script I needed to run to update the view on my live db.
I ran this script, re-ran the report, everything worked.