Enabling Shibboleth Attribute checker handler throws returns 404

[Peter Schober]

* Kumar, Prasanth (ELS-LON) <p.kumar.13 at elsevier.com> [2021-10-19 13:28]:
> The  error message seeing in the browser "The requested URL
> /Shibboleth.sso/AttrChecker was not found on this server".

Sounds to me you only performed the first step (adding the
'sessionHook' XML attribute to your ApplicationDefaults element) but
not the second (adding Handler XML element of type="AttributeChecker"
with the chosen Location="/AttrChecker").
-peter