No results when a key consists only of a backslash

Working on it.

@castanearie I tried to reproduce the issue, entered "foo \" in search but not encountered any issues. Here is the screencast.
If such problem still persists, can you provide the steps to reproduce the issue.

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.

Sorry, I don't have much knowledge about this module and its working, hence unassigning the issue from myself.

Automatically closed - issue fixed for 2 weeks with no activity.