Failed to execute 'observe' on 'MutationObserver': parameter 1 is not of type 'Node'

Developing with Concrete
1

Got a weird one, created a block in Block Designer on one site, worked fine, copied to another site and I get this error when clicking the Styles or “Headings” dropdowns in CKEditor:

Failed to execute ‘observe’ on ‘MutationObserver’: parameter 1 is not of type ‘Node’.

Please see error attached. Both sites v9, I updated the second site from v9.0.0 to v9.2.8 to match the first, but it hasn’t helped.

ckeditor-error
ckeditor-error3263×1313 470 KB

Has anyone encountered this and what was the fix?

Thanks

2

I’d guess that the server returned something other than JSON and that the real explanation is in your web server logs.