Problem:
When using Horde pages are not shown properly and in the apache error log you can find errors like that:
zend_mm_heap corrupted
Solution:
Seems that there has been a well known bug within PHP around for some years already (see http://bugs.php.net/bug.php?id=40479
Add following line in your Apache start up script (e.g. /etc/init.d/httpd)
export USE_ZEND_ALLOC=0