Something in your configuration is broken then.

A quick look at your backend configuration show that you enabled the 
null driver and included but didn't enable the email driver.

This is a support question, not a bug report.

Please use the mailing lists to ask for support.

http://www.horde.org/mail/ contains a list of all available mailing lists.

No, buttons like "Report as Spam" are not available in dynamic mode.

Does it work in dynamic mode?

The test system has a working Spamassassin configuration. Users should 
be able to report additional messages as Spam.

horde/imp/backends.local.php contains:

$servers['imap']['spam']['innocent'] = array(
              'display' => true,

              // Email reporting driver
              'email' => null,
              'email_format' => 'digest',

              // Null reporting driver
              'null' => true,

              // Program reporting driver
              'program' => null
          );
$servers['imap']['spam']['spam'] = array(
              'display' => true,

              // Email reporting driver
              'email' => null,
              'email_format' => 'digest',

              // Null reporting driver
              'null' => true,

              // Program reporting driver
              'program' => '/usr/bin/spamc -u mailfilter'
          );

If I click on the button "Report as Spam" in message view of Basic 
view I get this error message:

A fatal error has occurred
Call to a member function getFlags() 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 getFlags() on a 
non-object
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => 
/srv/www/vhosts/example.com/html/intern/horde/imp/lib/Basic/Message.php
     [line:protected] => 230
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [function] => catchFatalError
                     [class] => Horde_ErrorHandler
                     [type] => ::
                     [args] => Array
                         (
                         )

                 )

         )

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

The Horde log file doesn't contain entries about this error. The 
Apache error log contains:

[Fri Sep 11 21:30:25.362005 2015] [:error] [pid 12538] [client 
XXX.XXX.XXX.XXX:41162] PHP Fatal error:  Call to a member function 
getFlags() on a non-object in 
/srv/www/vhosts/example.com/html/intern/horde/imp/lib/Basic/Message.php on 
line 230, referer: 
https://www.example.com/intern/horde/imp/basic.php?mailbox=SU5CT1guc3BhbS5oaWdo&buid=170013&page=message