Problem with Relevanssi

When used with Relevanssi fatal errors happen on search pages:

[07-May-2017 01:41:28 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function json_encode() in /home/u707pt/public_html/wp-content/plugins/wp-super-cache/wp-cache-phase1.php:180
Stack trace:
#0 /home/u707pt/public_html/wp-content/plugins/wp-super-cache/wp-cache-phase1.php(343): wp_cache_serve_cache_file()
#1 /home/u707pt/public_html/wp-content/advanced-cache.php(20): include_once('/home/u707pt/pu...')
#2 /home/u707pt/public_html/wp-settings.php(84): include('/home/u707pt/pu...')
#3 /home/u707pt/public_html/wp-config.php(91): require_once('/home/u707pt/pu...')
#4 /home/u707pt/public_html/wp-load.php(37): require_once('/home/u707pt/pu...')
#5 /home/u707pt/public_html/wp-blog-header.php(13): require_once('/home/u707pt/pu...')
#6 /home/u707pt/public_html/index.php(17): require('/home/u707pt/pu...')
#7 {main}
  thrown in /home/u707pt/public_html/wp-content/plugins/wp-super-cache/wp-cache-phase1.php on line 180