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OK, that means there's some broken JSON in the params of one of your elements.

Unfortunately, the only way to get some clue as to which one is to modify some Joomla core code.

Edit the file:

./libraries/vendor/joomla/registry/src/Format/Json.php

... and around line 72 will be ...

Code (Text):

        if ($decoded === null)
        {
            throw new \RuntimeException(sprintf('Error decoding JSON data: %s', json_last_error_msg()));
        }
 
... modify that to add the actual data to the error message:

Code (Text):

        if ($decoded === null)
        {
            throw new \RuntimeException(sprintf('Error decoding JSON data: %s : the bad data is: %s', json_last_error_msg(), $data));
        }
 
That should give you some clue as to which one it is. Then of course it has to be fixed, and we have to work out how the bad data got in there.

So do that, and paste the new error message here.