Working with Joomla 3.6.5 and K2 2.7.1, I have run inrto a persistent error after importing older content from Joomla articles. When trying to access one of the K2 itmes, a JSON syntax error message appears and forces the user back to the main panel.

The solution is easy to implement as long as you have access to PhpMyAdmin in Cpanel (or some other Mysql database control panel GUI).

Looking at the 'metadata' column in K2_items table, I saw that the imported articles had a value of:

{"robots":"","author":"","rights":"","xreference":""}

While K2 items created 'natively' had a value of:

robots
=author=

Solution: in the SQL panel of PhpMyAdmin, run the following:

SELECT * FROM `prefix_k2_items` SET `metadata` = 'robots=author=' WHERE `metadata` = '{"robots":"","author":"","rights":"","xreference":""}'
Note: BE SURE TO REPLACE 'prefix' with the actual prefix used in your database.