🇦🇹Austria @castanearie

In a fresh Drupal install, I added the Search API Solr module, configured the Solr Server, added the Index default with Content as the data source and Body as its only field. Then I am able to cause the error with the following code.

$index = \Drupal\search_api\Entity\Index::load('default');
$query = $index->query();

/** @var \Drupal\search_api\ParseMode\ParseModeInterface $parse_mode */
$parse_mode = Drupal::service('plugin.manager.search_api.parse_mode')->createInstance('terms');
$parse_mode->setConjunction('OR');
$query->setParseMode($parse_mode);

$query->sort('search_api_relevance', 'DESC');

$query->keys('foo \\');
try {
  $query->execute();
}
catch (\Drupal\search_api\SearchApiException $e) {
  dpm($e->getPrevious()?->getMessage());
}

Both the parse mode of terms and sorting by search_api_relevance seem to be key to triggering the error.

castanearie → created an issue.