Weird insertion of part of the content in page edit window

When opening the page edit window in WordPress 5.3.2 with Natural Lite installed – part of the html head and the page content gets weirdly inserted directly into the body element.
The content will show in the Gutenberg Editor as well – where it can be edited as normal.
When saving the edits there will be a warning message:
Aktualisierung fehlgeschlagen. Fehlermeldung: Die Antwort ist keine gültige JSON-Antwort.
But it seems the changes are still saved and the new content is visible on the website.
So it’s kinda still working – but only for advance people – who can overlook and won’t be disturbed by the wrongly inserted content and warning messages.
Needs an urgend fix.
I did disable all plugins besides the one required by theme to check – but with no effect.

• This topic was modified 4 years, 9 months ago by samreciter.