Either Duplicate of #13264. Or you are using Debian/Ubuntu supplied 
PHP, which ships with a known broken version of json.

After text searching email through the quick search bar opening any of 
the results yields following error:

A fatal error has occurred
Call to a member function fetch() on a non-object

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] => Call to a member function fetch() on a non-object
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => 
/var/www/org/absurdistan/mail/horde/imp/lib/Contents.php
     [line:protected] => 1616
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [function] => catchFatalError
                     [class] => Horde_ErrorHandler
                     [type] => ::
                     [args] => Array
                         (
                         )

                 )

         )

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