If you get the following error when starting Apache:
apache2: Syntax error on line ...: Cannot load mod_proxy_http.so into server: mod_proxy_http.so: undefined symbol: ap_proxy_location_reverse_map
…then make sure that mod_proxy is enabled, and that it is loaded BEFORE mod_proxy_http. Apache doesn’t support dependency management of modules, so they have to be enabled in the appropriate order.
You might want to disable the modules and reenable them in the right order with a2enmod, or change the order in for instance /etc/sysconfig/apache2