What are the steps to reproduce your problem?
1. Create a User Task Duration, Aggregation Max report as table
2. Create a User Task Duration, Aggregation Min report as table
3. Combine the reports in a combined report
4. Try download as CSV
What is the problem?
I do not get a CSV, but instead a JSON in the UI which looks like an error.
What would be the expected behavior:
I can download the CSV for Combined Reports
The bug cause are user tasks that haven't been executed and thus their duration value is null. It not just affects combined reports but all reports.