An interesting Joomla update upset quite a few users - why ?
Here is the forum thread -> https://forum.joomla.org/viewtopic.php?p=3438726
So there was a check that was added to the new codebase and it stopped many people including myself on a few websites from accessing the backend! That's quite a serious thing.
It is line 72 in the file /libraries/vendor/joomla/registry/src/Format/Json.php this line.
Changing this line to the following tells you what the failing json is looking like. This then allows you to try and match that directly in the database for any extension or user that has bad JSON in it.
\RuntimeException(sprintf('Error decoding JSON data: %s the bad data is: %s', json_last_error_msg(), $data));
the $data is showing the JSON that is malformed. This needs to be replaced with "{}" without quotes.
select id, title, module, params from abcde_modules where params like '%{""}%';
That SQL enables you to locate matching modules with the rogue JSON. You need to replace the abcde with your database prefix. As you can see in the image I had just 4 - lucky and editing them to just {} allowed me to access again the backend of the site.
At Cambs Digital we do p and Wordpress development alongside Joomla development.