problem description: when logging in to horde and trying to go to the inbox you either get a blank page or a pop-up to save mailbox.php (can also be sidebar.php or others)
solution:
add export USE_ZEND_ALLOC=0 to /etc/apache2/envvars (on deb/ubuntu) or to /etc/sysconfig/httpd (red hat flavours)
restart httpd