Nice one Arthur :)

Hi friends, i have the similar problem. I have joomla 3.8.1 website with more than 1700 articles. I upgraded the page form joomla 2.5 to 3. Everything is working nice. I installed k2, imported articles and all works nice except in k2 admin i can not open any item the error is:
Manual editing is not an option. An deleting meta data is not an option, All articles has meta data.

I tried fix from gihub it does not helped. Core files is not an option.

what can i do?
Thanks

You can try the link here: www.joomlaworks.net/forum/k2-en/46914-0-error-decoding-json-data-syntax-error?start=20#163880

I tried following the link to www.robertwent.com/blog/fixing-json-data-errors-after-updating-to-joomla-3-6-3/ but the github site referenced did not work.
I did a little searching on my site using phpoMyAdmin.
I found that there were multiple articles that I could not open (throwing the error message in the post subject) that had issues with the metadata.
I used the find and replace feature of phpMyAdmin to search the metadata field and look for {"robots":"","author":"","rights":"","xreference":""},
in the replace field I added robots= author=.
Once replaced I am now able to open and edit the articles.

Hope this helps.

Dustin

Thanks Dustin :)

I have read through much of the posts on this issue and am having a hard time accepting that the problem is not with K2.

Here's why - Clean install of Joomla 3.8.5 with sample data and install only K2. The sample data has only one article. Import Joomla content into K2. Attempt to access the one article through K2 results in the error.

How can this be a Joomla problem?

Your solution solwing my problem.
Please, if you know, what will be correct SQL query to change all {"robots":"","author":"","rights":"","xreference":""},
to
robots= author=

Did you try this tool?
github.com/robwent/joomla-json-db-check

Unfortunately its not helped

Solution for Joomla above 3.8.x:

Problem is in this case is the "registry.php" File.
Path: libraries/vendor/joomla/registry/src/Registry.php

In the "zip"-File are 2 Files.
a.) original "registry.php" File, that causes the failure on K2 items that are imported.
b.) modified "registry.php" file, that does not cause the problem.

Here The Solution.

- open the zip in attachement with 2 Files. the original and the temporarily registry.php files.
- replace the original registry.php with the temporarily registry file
- then open all the K2-Items that causes this problem, and Save (or Save and close)
- important!! at last, replace the temporarily file with the original again.
---> because while you replaced the original with the temp-file, in front-page will be an error in this K2-Item.
But with the original file again, this problem wont exist.

hope i could help.