There are many tutorials available on the Internet, which are faulty or outdated. Apache version changes, Phusion Passenger changes (if used), underlying operating system changes, Ruby version, and so on. Any of these can cause Redmine to break or misbehave in some way.
We can group Apache problems into two groups:
Start-up problems (usually syntax error, or misconfigurations), preventing your Apache from normal startup
Runtime problems, and problems during Redmine startup via passenger or some other server if running as reverse proxy
To properly resolve your Apache problem, take the following steps: