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

Hints (optional):
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.