After updating joomla and all components to the latest version, the K2 stopped working fine. I can not add new article and edit existing articles. When
Clicking on the "new article" icon shows a white page. When I click on the "articles" I see a list of articles but I can not edit them

Joomla 3.6.5, K2 v 2.7.1.
Directory permissions - OK

Errors:
Strict Standards: Only variables should be assigned by reference in /home/racemoti/www/plugins/k2/approveandnotify/approveandnotify.php on line 90
Fatal error: Class 'JParameter' not found in /home/racemoti/www/plugins/k2/approveandnotify/approveandnotify.php on line 91

Hello,

The issue with a 3rd party K2 extension the "approve and notify".
You need to disable this extension and contact the extension's developer.

I also have this problem and I do not appear to have the extension "approve and notify"
Any ideas?
Joomla 2.5.11

Joomla! 2.5.11 is really outdated and insecure, you need to update to at least 2.5.28 and make sure to use K2's latest version.