Answer
Business Reporter has no way to automatically determine which sensitivity label a given published report should carry, so a report published without one can conflict with a mandatory-labeling policy and throw exactly this Excel-interop error when the report tries to manipulate the workbook. The workaround is to manually set the appropriate sensitivity label on each affected report, save it, and republish it — Business Reporter's Bulk Save and Republish features make doing this across many existing reports at once far more practical than handling them one at a time.