Updates params and rules with invalid default json entries. This will not fix errors from extensions which add invalid parameters.

joomla-json-db-check.php

<?php

//Initiate Joomla so we can use it's functions
/**
 * Constant that is checked in included files to prevent direct access.
 * define() is used in the installation folder rather than "const" to not error for PHP 5.2 and lower
 */
define('_JEXEC', 1);

define( 'DS', DIRECTORY_SEPARATOR );

if (file_exists(__DIR__ . '/defines.php'))
{
    include_once __DIR__ . '/defines.php';
}

if (!defined('_JDEFINES'))
{
    define('JPATH_BASE', __DIR__);
    require_once JPATH_BASE . '/includes/defines.php';
}

require_once JPATH_BASE . '/includes/framework.php';

// Instantiate the application.
$app = JFactory::getApplication('site');

$db = JFactory::getDbo();

$config = JFactory::getConfig();

$query = $db->getQuery(true);
$query->select('TABLE_NAME,COLUMN_NAME');
$query->from('INFORMATION_SCHEMA.COLUMNS');
$query->where('COLUMN_NAME = \'params\' OR COLUMN_NAME = \'rules\'');
$query->andWhere('TABLE_SCHEMA = \'' . $config->get('db') . '\'');
$db->setQuery($query);
$results =  $db->loadObjectList();

if ($results) {
    foreach ($results as $result) {
        echo "Checking table: {$result->TABLE_NAME}, column {$result->COLUMN_NAME}<br>";
        $query = $db->getQuery(true);
        $query->update($result->TABLE_NAME);
        $query->set($result->COLUMN_NAME . ' = "{}"');
        $query->where($result->COLUMN_NAME . ' = "" OR ' . $result->COLUMN_NAME . ' = \'{\"\"}\' OR ' . $result->COLUMN_NAME . ' = \'{\\\\\"\\\\\"}\' ');
        $db->setQuery($query);
       // echo ($query->__toString());
        $results = $db->execute();
        $changes = $db->getAffectedRows();
        if($changes != 0) {
            echo $changes . " rows modified.<br>";
        }
    }
}
?>

Take a backup of your database.
Upload the file to the root of your site and view it in a browser at http://yoursite.com/joomla-json-db-check.php
Check the site to see if the errors have gone.

Updated file with full db check here: https://github.com/robwent/joomla-json-db-check

@robwent thanks 👍 This saved my day!