Omeka has encountered an error

Apache_Solr_ParserException

Solr response does not appear to be valid JSON, please examine the raw response with getRawResponse() method

exception 'Apache_Solr_ParserException' with message 'Solr response does not appear to be valid JSON, please examine the raw response with getRawResponse() method' in /var/www/discover_production/plugins/SolrSearch/lib/solr-php-client/Response.php:206
Stack trace:
#0 /var/www/discover_production/plugins/SolrSearch/lib/solr-php-client/Response.php(164): Apache_Solr_Response->_parseData()
#1 /var/www/discover_production/plugins/SolrSearch/controllers/ResultsController.php(70): Apache_Solr_Response->__get('response')
#2 /var/www/discover_production/application/libraries/Zend/Controller/Action.php(516): SolrSearch_ResultsController->indexAction()
#3 /var/www/discover_production/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('indexAction')
#4 /var/www/discover_production/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#5 /var/www/discover_production/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#6 /var/www/discover_production/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#7 /var/www/discover_production/application/libraries/Omeka/Application.php(79): Zend_Application->run()
#8 /var/www/discover_production/index.php(23): Omeka_Application->run()
#9 {main}