ApplicationOverride results in "Unable to locate an encrypted key"

[Peter Schober]

* Cameron Kerr <cameron.kerr at otago.ac.nz> [2016-11-03 03:19]:
> After a day of struggling, I found the error; it was in httpd. I had
> 'ShibRequestSetting applicationId siteA' constrained to <Location
> /secure>, and some PHP stuff, but that was not enough; it didn't
> cover /Shibboleth.sso, thus when it went to siteA's
> /Shibboleth.sso/... mod_shib was rightly expecting a response
> addressed to the default entityID.

Either you have the override for the whole vhost (in which case
/Shibboleth.sso trivially would be covered anyway) or you'd need to
set a separate handlerURL for path-based overrides (i.e.,
/foo/Shibboleth.sso when /foo/ is the override)
So I don't really understand how you'd run into that situation.
-peter