Roundcube, php 5.6 and self-signed certificates

I am currently porting one of my servers to another. That server is running Postfix + Dovecot using a self-signed certificate. My web-frontend used to be roundcube. Now is the problem, that the new server has PHP 5.6 installed, which has known problems using self-signed certificates.

First of all, the error of roundcube is:

error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed in /var/www/roundcube/program/lib/Roundcube/rcube_imap_generic.php on line 915
[05-May-2015 13:11:11 +0200]: <8h4tkjbn> IMAP Error: Login failed for XXXXXXXXXXX from XXXXXXXXXXX. Unable to negotiate TLS in /var/www/roundcube/program/lib/Roundcube/rcube_imap.php on line 198 (POST /?_task=login?_task=login&_action=login)

I found, that you may put this in the config.inc.php:

$config['imap_conn_options'] = array( 
  'ssl'         => array( 
     'verify_peer'  => true, 
     'allow_self_signed' => true,
     'verify_depth' => 3, 
     'cafile'       => '/etc/postfix/sslcert/mailserver.crt', 
   ), 
 );

 $config['smtp_conn_options'] = array( 
   'ssl'         => array( 
     'verify_peer'  => true,
     'allow_self_signed' => true,
     'verify_depth' => 3, 
     'cafile'       => '/etc/postfix/sslcert/mailserver.crt', 
   ), 
 );

(Location of the .crt and .key file is /etc/postfix/sslcert/mailserver.*)

When putting this in the config, error is:

[05-May-2015 13:13:48 Europe/Berlin] PHP Warning:  stream_socket_enable_crypto(): Unable to locate peer certificate CN in /var/www/roundcube/program/lib/Roundcube/rcube_imap_generic.php on line 915

Ive tried various variations of this config lines, but I am stuck here. Maybe someone here could help me out!

If you need ANY further information, please state a comment and I will edit the first post!