/etc/apache2/conf.d/proxy.conf appears to be loading some modules in the wrong order
The apache default installation with apache2 and apache2-proxy packages fails to start when loading some modules from proxy.conf.
The lines in question are:
LoadModule lbmethod_bybusyness_module
modules/mod_lbmethod_bybusyness.so
LoadModule lbmethod_byrequests_module
modules/mod_lbmethod_byrequests.so
LoadModule lbmethod_bytraffic_module
modules/mod_lbmethod_bytraffic.so
LoadModule lbmethod_heartbeat_module
modules/mod_lbmethod_heartbeat.so
From my understanding, these modules depend on either mod_proxy.so or
mod_proxy_balancer.so, but they’re being loaded before them in the
proxy.conf file.
Moving those 4 lines to the end of the file solves the problem and
apache starts normally.
(from redmine: issue id 9326, created on 2018-08-22, closed on 2019-05-04)