apache2: Could not reliably determine the server's fully qualified domain name
Solution 1:
check your /etc/hosts file. It should look something like this:
127.0.0.1 localhost
a.b.c.d www.example.com
The first line should already be there, you only have to add the second line with your values.
Solution 2:
I think it's because you need to have "ServerName" declared outside vhost configuration, in the apache2.conf and the ServerName value declared in the /etc/hosts