This is not a Horde issue.  HTTP transfer details are handled by the 
HTTP server, not Horde/PHP.

We're running both latest H4 and Horde 1.2.9 on SLES11SP1 Linux with 
Apache2, set up so that client-side certificates are required to 
access the Horde directories.

Both directories (installed in sibling directories) are set up 
identically in the httpd config file.

When uploading attachments to new emails in Horde 1.2.9, this works 
for all file sizes.

When uploading attachments in Horde4, this only works for small files 
(up to about 5.4 kB upload works, 5.5 kB and more stalls). With the 
larger files, the message "Hochladen... (meindateiname)" 
("Uploading... (theuploadsfilename)") remains forever.

Looking at the httpd access and error log, I see the following:

- Horde 1.2.9, any file size, just access log, no error log entry:
ourserver - /mycertDNgoes here [27/Feb/2012:12:14:53 +0100] "POST 
/horde-webmail-1.2.9/dimp/compose.php HTTP/1.1" 200 177 
"https://ourserver/horde-webmail-1.2.9/dimp/compose.php?type=new" 
"Mozilla/5.0 (X11; Linux x86_64; rv:9.0.1) Gecko/20100101 Firefox/9.0.1"

- Horde4, small file, just access log, no error log entry
ourserver - /mycertDNgoeshere [27/Feb/2012:12:13:25 +0100] "POST 
/horde-webmail-4.0/services/ajax.php/imp/addAttachment HTTP/1.1" 200 
197 
"https://ourserver/horde-webmail-4.0/imp/compose-dimp.php?type=new&ajaxui=1&uniq=1330341182271" "Mozilla/5.0 (X11; Linux x86_64; rv:9.0.1) Gecko/20100101 
Firefox/9.0.1"

- Horde4, file 5.5 kB or larger:
access log:
ourserver - /mycertDNgoeshere [27/Feb/2012:12:12:41 +0100] "POST 
/horde-webmail-4.0/services/ajax.php/imp/addAttachment HTTP/1.1" 200 - 
"https://ourserver/horde-webmail-4.0/imp/compose-dimp.php?type=new&ajaxui=1&uniq=1330341126841" "Mozilla/5.0 (X11; Linux x86_64; rv:9.0.1) Gecko/20100101 
Firefox/9.0.1"

error log:
[Mon Feb 27 12:12:41 2012] [error] [client ourclient] Re-negotiation 
handshake failed: Not accepted by client!?, referer: 
https://ourserver/horde-webmail-4.0/imp/compose-dimp.php?type=new&ajaxui=1&uniq=1330341126841

This might be related to some Apache configuration issue - but since 
1.2.9 works like a charm and H4 doesn't, from the same client to the 
same server with identical configuration (except for directory names), 
I'd at least like to know what's different between both mechanisms.

One could of course say: Switching to H4 breaks it. ;-)

PS: Just in case someone might as, despite it working in 1.2.9:

/etc/php5/apache2/php.ini:
upload_max_filesize = 200M
post_max_size = 256M

And setting "LimitXMLRequestBody 0" in the horde4-related section of 
httpd's config doesn't bring any change, either.