apache: AH01630: client denied by server configuration

Browser showed an error:

403 Forbidden
You don't have permission to access /fullscreen.html on this server.
Apache/2.4.18 (Ubuntu) Server at conversejs.foo Port 80

Apache error log:

[authz_core:error] [pid 23] [client 127.0.0.1:2342] AH01630: client denied by server configuration: /home/cweiske/php/conversejs

The issue was that the document root directory did not exist. It should have been a 404, not a 403.