Since apache fails to stop gracefully, try to kill it with
# killall -9 httpd
Then try to start it. In case of failure please post latter lines from /var/log/httpd/error_log
Most probably this error occurs due to soft-failed Horde upgrade. AFAIK starting from Horde 3.2 they reimplemented logging solution, so if not all files were replaced you might suffer from compartibility problems.
As an instant solution I would advise you to disable logging, change setting to...
It may be related to inconsistancy of password and password policy.
You may either try to set more complex password while creating maillist or try to decrease Minimal password Strength at Home -> Settings -> Mail Server Settings to Very Weak.
Hope it will help =)