Upgrading to the June 2, 2020 release of Search API (8.x-1.17) causes "Call to undefined function theme_search_api_server()" errors in drupal-watchdog.log when used in conjunction with Acquia Search.
Upgrade Acquia Connector module to https://www.drupal.org/project/acquia_connector/releases/8.x-1.22 or higher.
Cause (this is optional)
This is a known issue as Search API module decided to change the theme subsystem which Acquia Search module uses. See https://www.drupal.org/node/3121637