Checked with tcpdum.  It was web server segfaulting when sidebar was 
requested.  Have no idea why does it segfaults...  I'll check if there 
are any (newer) patches for Apache and/or PHP, and see if some 
worarounds will work.

I'll check out possibility that server was segfaulting later in the 
day (don't have shell access to it from here).  I doubt it is, since 
currently I'm trying to access it using proxy server, and I don't see 
anything in proxy logs either (also, I'd get "zero sized response" 
error from squid proxy if it was my web server crashing).  I'll also 
try defining webmail as virtual host in Apache config, and see if that 
would work as workaround.  I'll report what I found and any success 
and/or failure (if nothing else, than for documentation if somebody 
gets into same trouble as myself).

In this case it doesn't have anything to do with the server name at 
all. Your server is probably segfaulting when generating the sidebar, 
this is why you don't see a request in your logs. Only finished 
requests are being logged.

I'd disagree that this is due to broken server configuration.



The name displayed in browser's URL is reachable.  The server can be 
reached using several different names (aliases), which is common and 
perfectly legal configuration.  All the server's aliases are reachable 
from the client (including the name server knows itself as).



If the name in URL was not reachable, I wouldn't be able to get login 
prompt.  I am able to get login promp.  I am able to login.  The 
content of main frame is displayed.  I can even access applications 
(such as IMP) by typing its name directly in brosers's URL.  All URL 
links in main frame are generated correctly.  However, the left frame 
(that should contain sidebar) is not loaded.



I also have UseCanonicalName set to off for this precise reason, so 
that server uses for its server name whatever client accessed it as.   
What is why there is UseCanonicalName for.  Again perfectly legal 
configuration.



Plus I made setup change in horde to hardcode host name it is supposed 
to use for generating URLs, as described in bug report).



However, horde is generating HTML that client is not able to follow 
(in Apache log files I don't see any attempt from client to load 
sidebar frame).

If your web server's name isn't the one that's displayed in the 
browser's URL, or isn't at least reachable at this URL, you have a 
broken Apache configuration.

I've just tested this with Internet Explorer.  Got same results as 
with FireFox.  Seems to be browser independent.

Hmmm....  Seems that this happens only if I steup Horde to use IMP as 
authenticatin mechanism.  In case when I setup Horde to use (for 
example) IMAP, this doesn't occur.  Generated HTML in both cases looks 
the same (sidebar URL doesn't have hostname), but when using IMAP for 
Horde auth, it is displayed.  Strange...

I encountered a problem where depending on Apache configuration, 
sidebar is not displayed.  The reason is that URL for the sidebar is 
missing the hostname (or at least that seams to be the problem).



<frame name="horde_menu" src="/horde-3.0.3/services/portal/sidebar.php ...>

<frame name="horde_main" 
src="https://webmail.milivojevic.org/horde-3.0.3/ ...>



The problem is there when hostname of the server doesn't match the 
name in URL.  For example, my host's name is svarozic, and I have 
CNAME webmail and www for it.  The name svarozic is not visible from 
Inernet (only visible on internal network).  In Apache's configuration 
file I have ServerName unset, and UseCanonicalName set to off.  In 
Horde's configuration I have following:



$conf['server']['name'] = 'webmail.milivojevic.org';

$conf['cookie']['domain'] = 'webmail.milivojevic.org';



The browser I was using was Mozilla Firefox 1.0.