[imp] Imp Filter error - Class 'IMP_Imap_Ftree_IteratorFilter' not found - Horde_ErrorHandler::catchFatalError()

[info at interpost.biz]

Imp failed to rename the sent mailbox on the first of the month and  
threw the following error (We are using php 5.5.16 on this server):
----------------------------------------------------------------------

Class 'IMP_Imap_Ftree_IteratorFilter' not found

1. Horde_ErrorHandler::catchFatalError()
Details

The full error message is logged in Horde's log file, and is shown  
below only to administrators. Non-administrative users will not see  
error details.

ErrorException Object
(
     [message:protected] => Class 'IMP_Imap_Ftree_IteratorFilter' not found
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] =>  
/var/www/horde/imp/lib/LoginTasks/Task/DeleteSentmailMonthly.php
     [line:protected] => 45
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [function] => catchFatalError
                     [class] => Horde_ErrorHandler
                     [type] => ::
                     [args] => Array
                         (
                         )

                 )

         )

     [previous:Exception:private] =>
     [severity:protected] => 1
     [logged] => 1
)

---------------------------------------------------------------

Sent mailbox was not renamed.

Best

Ed