[horde] User not authorized for horde

[Stuart C. Naifeh]

I'm getting this error message for all of my non-admin users.  I've googled
the message and seen a lot of discussion if similar issues, and I've tried
the various solutions proposed, but none of them work.  My set up is as
follows:

Ubuntu 12.04
Apache 2.2
php 5.3
Horde 5.0.4 installed from Pear

I have horde installed at /var/www/horde

In my server setup on Apache, if have an Alias directive pointing /webmail
at /var/www/horde (so horde is at https://www.myserver.com/webmail')

I have $conf['cookie']['path'] set to '/webmail'

I use LDAP for Horde authentication, and all my users are in the LDAP
database, and they can be authenticated to dovecot and postfix using LDAP
authentication.  In my Imp prefs, I have 'hordeauth' set to 'full'.

I can authenticate users listed in horde as admins no problem, but for
regular users, the log looks like this:

[horde] Login success for joeuser [111.222.333.444] to horde. [pid ### on
line 163 of "var/www/horde/login.php"]
[imp] Login success for joeuser (Horde user joeuser) [111.222.333.444] to
{localhost:143 [imap]} [pid ### on line 183 of
"var/www/horde/imp/lib/Auth.php"]
User joeuser is not authorized for Horde. [pid ### on line 275 of
"usr/share/php/Horde/Registry.php"]

If I add joeuser to the list of admins, he can log in just fine, but when I
take him out again, no dice.

I tried changing the cookie path to "/" as one user suggested, I've of
course tried deleting cookies before attempting the login as a different
user, and I've checked and rechecked my config/prefs files, but I don't see
anything wrong.

Can anyone tell me what's wrong with my setup?