Fresh instal of K2 in current version of Joomla (Joomla 3.9.21, K2 Version 2.10.3)
Everything works except when I go to add an image to an item, and browse the server. The media window appears, then I get an error

Invalid backend response.
Data is not JSON.,

But instead of it ending with just that, what I actually have is a complete page (screen shown below). I deleted the assets folder and ran the K2 install again but no change.

ok error reporting set to none seems to clear the issue, so the question is, will this be a K2 plugin or some other one?

Thanks by the way