Error when searching for Participants
https://civicrm.stackexchange.com/questions/24671/error-when-searching-for-participants
It seems that there is regression because of https://github.com/civicrm/civicrm-core/pull/11706
Commenting line 4912(self::getGroupByFromOrderBy($groupBy, $orderBys);) in CRM/Contact/BAO/Query.php seems to fix the issue.