Try each of the following in order:
1. If using a saved report, ensure all formulas on the columns tab of the report are valid.
- For example, if the report at one time had 10 columns, but now only has 9, ensure none of the formulas reference the non-existant column.
2. If columns were recently deleted, save and close and reopen the report
3. Ensure all filters on the filters tab and filters tabs of the columns are valid.
4. Recreate the report from scratch