[turba] Apache2 segfault in turba add contact

[maudam]

Hi,
when /horde/turba/add.php page is called from turba i got a segmentation 
fault in apache2.
[Mon Jan 25 10:12:39.469522 2016] [core:notice] [pid 22609] AH00052: 
child pid 22685 exit signal Segmentation fault (11)

Digging in add.php file and setting some php exit() i found the trouble 
is here:

Line #63
/* Set up the form. */
$form = new Turba_Form_AddContact($vars, $contact);

All horde and turba packages are installed and updated using PEAR on a 
debian 8 system.

PHP Version 5.6.17-0+deb8u1
Horde Groupware 5.2.11

How can i solve this issue ?
Thanks in advance