Can't view contact groups page: "DataTables warning"
In this case, turning on logging (civicrm/admin/setting/misc?reset=1) and looking in the CiviCRM log reveals a fatal error related to mapping and geocoding:


  Proximity searching requires you to set a valid geocoding provider


Somebody had tried to set up a proximity group based on zip codes, but the mapping and geocoding settings had not been configured. Configuring the geocoding (civicrm/admin/setting/mapping?reset=1) to use Google as the provider fixed the issue.