To replicate, search all contacts, try to scroll to page 2 "Network Error Unable to reach the server. Please refresh this page in your browser and try again."
Reported by 2 clients, able to replicate. Workaround is to use the 'glossary' of first letter of display name. Pagination works fine after this. Reverting to 'All' also reverts the problem
Replicated on CiviCRM v 5.11.0 and 5.12.0 Wordpress 5.1.1 and Drupal 7 Ubuntu 16.04 PHP 7.1.24, Apache