Have been extremely busy the past few months, so accept my apologies. But since this isn't going to change anytime soon, all I can say is that you'll have to do some digging on your own. E.g. examine the JSON (XHR) response using your browser's console. It's most likely some 3rd party system plugin injecting bad code onto the JSON output of K2. It can be a PHP notice for example. In most cases it's easy to double check by setting error reporting to none in Joomla's Global Configuration.

An update on the matter.

It has come to my attention that Plesk, the hosting control panel, will block the CSS/JS files of older elFinder releases. elFinder is the 3rd party media manager that is bundled with K2 (and SIGPro etc.).

This is probably the reason why you see a blank page (if it's not a JSON issue of course as I indicated above).

Please confirm that your site is hosted on Plesk for those with this issue.

I will upgrade elFinder in its latest release in a while and I'll update this thread in the coming hours (or sooner) so you can install the 'dev' release of K2.

K2 has been updated to the latest elFinder release, so for anyone with K2 on a Plesk server, this should resolve your issues.

You can now download K2 v2.11 (dev) from: getk2.org/downloads/?f=K2_Development_Release.zip