1) Keep the default zone apache.conf without the ScriptAlias.

2) Stop both zoneminder and apache2
sudo service apache2 stop
sudo service zoneminder stop

2) Remove any stale sockets from /tmp/zm (default location):
sudo rm -vf /tmp/zm/*.*

3) sudo a2enmod cgi

4) Start zoneminder
sudo service zoneminder start

5) Start apache2
sudo service apache2 start

I found out that the order was important for step 4 and 5, otherwise it doesn’t work

Source: Bug #1159361 “zoneminder not creating local unix sockets for str…” : Bugs : zoneminder package : Ubuntu