panels/ajax/editor/style-settings/panel_context error

I'm trying to use the module panels_extra_styles and add wrapper:raw styling to a pane.
I get the following error:

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: https%3A//xxx/panels/ajax/editor/style-settings/panel_context%253Anode_view%253Anode_view_panel_context/region/e_
StatusText: OK
ResponseText:
Node template | xxx
@import url("https://xxx.com/modules/system/system.base.css?mha2h4");
...
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I don't think it is a panels_extra_Styles module issue as I have it working fine on another site. I've tried to disable the admin toolbar, check 'JS aggregate functions' and other solutions suggested on posts but nothing is working.
How do I get rid of this error?
Thanks.