If you face this problem while creating the sent folder in horde on a linux Plesk server 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 and hence you were getting that error every time you try to create a sent folder.
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 refresh your Web-Browser and then try to create a sent folder by following the below steps :
Click on Options and then, within the Mail category, click on Personal Information.
Scroll down to the bottom of the page, and click on the save sent mail ? checkbox and finally click on save options.