Whatever the problem was, it seems to be fixed as of version 1.2.9 of 
webmail edition.

Thanks.

Those mailbox.php calls make no sense.  IMP's compose screen doesn't 
do anything to refresh the calling (parent) screen.  So not sure where 
that is coming from.

You should probably use something like Firebug to track the actual 
data being passed to/from the server.

I can't tell that the script is loading at all after I hit either 
"Send Message", "Save Draft", or "Cancel Message".  If I just add an 
"echo 'blah';" command at the start of the script, then I never see my 
'blah', just a blank screen.

Perhaps another clue, though not sure what it means since I don't know 
how imp is working... in my apache logs, if I'm on a windows box using 
either IE or FF, if I navigate to the "New Message" screen then click 
"Cancel Message", I consistently see 3 hits on the server, something 
like this:

192.168.0.187 - - [30/Oct/2010:21:34:04 -0500] "GET 
/imp/mailbox.php?mailbox=INBOX HTTP/1.1" 200 17221
192.168.0.187 - - [30/Oct/2010:21:34:05 -0500] "POST 
/imp/compose.php?uniq=3sk5f8g600o HTTP/1.1" 302 26
192.168.0.187 - - [30/Oct/2010:21:34:05 -0500] "GET 
/imp/mailbox.php?mailbox=INBOX HTTP/1.1" 200 17221

BUT, if I'm on my fedora box (in Firefox of course) and do the same 
sequence, I only see one hit on my server:

192.168.0.186 - - [30/Oct/2010:21:01:49 -0500] "POST 
/imp/compose.php?uniq=119rbuz1teyo HTTP/1.1" 200 6855

Note the different response codes (302 when all works versus 200 when 
it doesn't) for the imp/compose.php script.

Hopefully this helps someone figure this out. I'm happy to do more 
testing if anyone has ideas.

Probably will need to manually enter PHP debug statements in 
imp/compose.php to determine where in the script it is breaking and/or 
whether the script is being loaded at all.

I have read the FAQ and scoured the wiki, bug tracking, and google to 
no avail, including http://wiki.horde.org/FAQ/Admin/Troubleshoot#wsod.

I am using Horde Groupware Webmail Edition, version 1.2.7. All works 
perfectly under Windows using IE or Firefox.

However, under Fedora 9 and above (haven't tested before that) when 
trying to send an email or save a draft of an email, the screen 
blanks. No email message is sent nor saved. There is no error in my 
web logs nor in horde logs. There is no segfault generated in server 
logs (/var/log/messages). If I turn error reporting level in horde all 
the way up, I only get usual stuff about checking for alarm and memory 
used report. I have memcache disabled and cache is set to not be used.

If anyone has any further ideas about how to investigate, I welcome them.

Thanks in advance.