Getting this message in Media Manager and unable to use Media Manager. I'm using Joomla 3.9.14 and K2 2.10.2.

Any fixes available?

Removed assets folder and reinstalled K2. Permissions on the images folder look OK.
Still getting the message: Invalid backend response. Data is not JSON. The most recent request was denied because it had an invalid security token. Please refresh the page and try again.

Error reporting is already switched to 'None'.
I've cleared server and browser caches.

Browser developer console shows 2 messages:
This page uses the non standard property “zoom”. Consider using calc() in the relevant property values, or using “transform” along with “transform-origin: 0 0”. index.php

JQMIGRATE: Migrate is installed, version 1.4.1 jquery-migrate.min.js:2:552

The first is from administrator/index.php.
The second from media.jui/js/jquery-migrate.min.js
Neither is an error and I don't suppose they are the problem.

I don't see anything on the logs (assuming I'm looking at the correct ones. The installation is controlled via CPanel/Softaculous).

Apologies for lack of response - my original reply was trashed in a power outage, and subsequently I've been away.

In administrator/index.php line 490:12 the following URL is invoked:
url : '/test/j3/administrator/index.php?option=com_k2&view=media&task=connector',
height: 600

and this fails with reported message - I'm not seeing a Response tab as the error is clearly inside the the K2 Media module. Can you be more specific in the steps to follow from this point.

Thanks

OK _ found the response tab.

URL: test/j3/administrator/index.php?option=com_k2&view=media&task=connector&cmd=open&target=&init=1&tree=1&_=1581872988632

returned the message 'The most recent request was denied because it had an invalid security token. Please refresh the page and try again.' in plain text (i.e. no HTML)