The perf-o-meter is throwing a "Cannot modify header information - headers already sent " on SLE_15, see the attached image, for 1.6.0p8cre. I’m not seeing the “space” before|after the PHP brackets which is usually related to this type of issue.
This error comes not from the perf-o-meter this is generated if you click the perf-o-meter and the graph should be rendered.
You can first try to apply the fix from this post for the JSON errors.
If your error persists after the change, the new error message would be interesting.
that resolved the issue, thanks