This has been fixed already a year ago.

I report that the patch here fix the problem.

Kinglok, Fong

I report the same problem after employing hook (authusername) to strip
off domain name when loggin horde and kronolith (4.1.1).

When I try using kronolith and obtain the subscription URL of CalDAV,
it shows:
https://mail.domain.com/rpc/principals/foo@domain.com/

Then, I try adding the account through iCal (OSX10.8.4) with using the
URL and username as 'foo@domain.com' and it shows 'Authentication
failed. Your username and password were rejected by the server'.  The
horde logging shows nothing about it.

The weird point lies in when I using the following URL to add account:
https://mail.domain.com/rpc/principals/foo/

and using 'foo' as username, iCal can add the account successfully.
However, there is simply no calendar shown.

On the contrary, if I disable the hook.php and caldav in horde work
flawlessly with horde.  I have two calendars in horde site and they
are shown in iCal.  When I enable the hook.php again and re-add the
account again in ical no calendar can be shown.

Therefore, I believe there are some problems in using the hook and
CalDAV user name.

Kinglok, Fong

In my horde-setup (Horde-5.1.1, IMP-6.1.1, Horde_Dav-1.0.0) I am using 
horde-usernames of form 'user@domain' and auth-usernames of form 'user'.

This leads to problems with CalDav-access: if I access a caldav-URL 
with the browser, I get the (xml-)message 'User user@domain does not 
exist'. This is related to the authentication-routine 
getPrincipalByPath($path)  in 'Horde/Dav/Principals.php', where the 
horde-username from the URL is used as auth-username. It can be fixed 
by converting the username to auth-username (see attached patch).

This leads to the new error message 'User did not have the required 
privileges ({DAV:}read) for path "..."', as the authentication in the 
browser has been done using the auth-username, which is used in the 
routine 'getCurrentUserPrincipal()' in 'Sabre/DAVACL/Plugin.php' to 
set the principal-path, which should contain the horde-username. This 
can be fixed likewise by converting the username to horde-username 
(see second patch).

Is my observation and the fixing correct, or do I miss something here?

Thanks for your help,
regards,
Andreas