Sometimes, when you try to login to horde webmail client and you are getting a blank page at first until you refresh your browser.
This means that the php has been upgraded to 5.3.6 or later version on that server and some of the horde webmail namespaces are not compatible with the php 5.3.6 version.
As a work around you need to edit the below two files to fix this error :
/usr/share/psa-horde/imp/lib/IMAP/Client.php line 576
function namespace($additional = array())
function plnamespace($additional = array()
/usr/share/psa-horde/imp/lib/Session.php line 295
$_SESSION[‘imp’][‘namespace’] = $imapclient->namespace($user_namespace);
$_SESSION[‘imp’][‘namespace’] = $imapclient->plnamespace($user_namespace);
Restart the httpd service on the server.
Now, just logout and login once again and you will be able to login to horde at the first attempt only.