Can you check that the PHP version Piwik Matomo is using is the same as the one with php-json enabled. You could e.g. upload a phpinfo-file to the piwik directory and check there.
Keep in mind that Piwik 3 needs at least PHP 5.5.9.
Thanks Likas,
Everything apparently is installed correctly, but I realized that I hadn’t restarted php-fpm after the the install of the json module.
php -m said the module exists, but your comment about phpinfo revealed the JSON module didn’t get a section on this html output.
now all works, thanks!