Unable to send attachments in Horde

I recently came across a bizzare issue in the version of Horde webmail shipped with Parallels Plesk 9.5.2 where it would allow you to attach files to an e-mail you are composing, but when you send the e-mail the attachment never arrives. Sending the same attachment via the Atmail webmail client or a traditional SMTP client wasn’t a problem and the mail server logs didn’t indicate that the attachment was getting stripped, so it was eveidently never getting attached to the e-mail in the first place.

For some reason Parallels Plesk doesn’t specify a seperate ErrorLog for the Horde webmail.domain.com vhost in /etc/httpd/conf.d/zzz_horde_vhost.conf so all of the logs from Horde end up in the main Apachce HTTPD error log at /var/log/httpd/error_log.

In the logs there should be PHP errors where Horde is unable to load one of it’s shared libraries that is used for processing attachments:

PHP Fatal error: Can’t load Horde/Serialize.php, open_basedir restriction. in /usr/share/psa-horde/lib/Horde/SessionObjects.php on line 134,

The easiest way to fix this is to change the relative path to the file (Horde/Serialize.php) to an asbolute path, so take a copy of /usr/share/psa-horde/lib/Horde/SessionObjects.php as a backup then open the file up and
look for two lines that say:

require_once ‘Horde/Serialize.php’;

And edit these two lines to have the full path:

require_once ‘/usr/share/psa-horde/lib/Horde/Serialize.php’;

Now all you have to do is save the file and test that Horde has started working as expected. Remember that future updates to Horde from Parallels will overwrite this file and you will have to re-apply your change.