How to fix this JSON Error with Joomla 3.7, CiviCRM 4.7.19, PHP 7.1?
As of May 2017, I'd say PHP 7.1 / CiviCRM combination is probably not ready for production use - I tried it the other day and ran into some errors. If you need to run CiviCRM in production, I'd switch down to PHP 7.0.x or 5.6.x.

You might be able to get it to behave with some other configuration changes or by being more actively engaged in CiviCRM development, but for a production site I'd recommend shifting down to 7.0.x.