Bar chart is empty (Moodle 4)

[abautu]

I use configurable reports on a Moodle 4 instalation. It is working well, except that bar charts result in an empty response (i.e. loading the image results in an HTTP response with code 200, type image/png, but content-length is 0. I found that graph.php cleans the output buffer (using ob_clean) after rendering the chart data (thus throwing it away). This is in contrast with pie and line charts, which call ob_clean before the final image stroke (to remove any unwanted text, but still render the image).