AH01753: access check of ‘127.0.0.1’ to /xxx/ failed, reason: unable to get the remote host name

This error message can be caused by placing an IP instead of a hostname in a Require host statement in Apache 2.4+. After porting some old access rules to Apache 2.4 I had used Require host 127.0.0.1 instead of the correct Require ip 127.0.0.1. Switched it, and ahoy! It now works.