ES 2.3 Error optimising and rebuilding search index with Enhanced Search enabled help

XFES\Elasticsearch\RequestException: Unknown error (HTTP code: 400) in src/addons/XFES/Elasticsearch/Api.php at line 466
XFES\Elasticsearch\Api->request() in src/addons/XFES/Elasticsearch/Api.php at line 335
XFES\Elasticsearch\Api->deleteIndex() in src/addons/XFES/Service/Optimizer.php at line 61
XFES\Service\Optimizer->optimize() in src/addons/XFES/Admin/Controller/EnhancedSearch.php at line 196
XFES\Admin\Controller\EnhancedSearch->actionOptimize() in src/XF/Mvc/Dispatcher.php at line 362
XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 264
XF\Mvc\Dispatcher->dispatchFromMatch() in src/XF/Mvc/Dispatcher.php at line 121
XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 63
XF\Mvc\Dispatcher->run() in src/XF/App.php at line 2826
XF\App->run() in src/XF.php at line 806
XF::runApp() in admin.php at line 15

XFES\Elasticsearch\RequestException: Unknown error (HTTP code: 400) in src/addons/XFES/Elasticsearch/Api.php at line 466
XFES\Elasticsearch\Api->request() in src/addons/XFES/Elasticsearch/Api.php at line 335
XFES\Elasticsearch\Api->deleteIndex() in src/addons/XFES/Service/Optimizer.php at line 61
XFES\Service\Optimizer->optimize() in src/addons/XFES/Search/Source/Elasticsearch.php at line 145
XFES\Search\Source\Elasticsearch->truncate() in src/XF/Search/Search.php at line 175
XF\Search\Search->truncate() in src/XF/Job/SearchRebuild.php at line 29
XF\Job\SearchRebuild->run() in src/XF/Job/Manager.php at line 275
XF\Job\Manager->runJobInternal() in src/XF/Job/Manager.php at line 205
XF\Job\Manager->runJobEntry() in src/XF/Job/Manager.php at line 121
XF\Job\Manager->runByIds() in src/XF/Admin/Controller/ToolsController.php at line 146
XF\Admin\Controller\ToolsController->actionRunJob() in src/XF/Mvc/Dispatcher.php at line 362
XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 264
XF\Mvc\Dispatcher->dispatchFromMatch() in src/XF/Mvc/Dispatcher.php at line 121
XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 63
XF\Mvc\Dispatcher->run() in src/XF/App.php at line 2826
XF\App->run() in src/XF.php at line 806
XF::runApp() in admin.php at line 15