As discussed here:

When using the tracker import feature (under the tracker's settings) with an invalid json.gz file (for example, an invalid "resolution" value), there's no output on what was wrong with the uploaded file. The only effect is that the message "An import is in progress" doesn't appear and the tickets newer show up.

This lack of error message(s) makes it very difficult to fix the JSON file. I could only do this by systematically removing or changing data in the JSON file to see when it would be imported or not.