SEARCH API + VIEW to search members
You can look into this : 
https://api.drupal.org/api/drupal/includes!database!database.inc/function/db_query/7#comment-7019

https://api.drupal.org/comment/51338#comment-51338

https://api.drupal.org/comment/16974#comment-16974

And check if you made change in your taxonomy vocabulary, deleting any fields that the search would index in the views. 

You can also re-index your user.