If you got:

Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration

message in error log of your apache2 web server under Ubuntu you need to do next:

Run this command in terminal with sudo

cp -arp /etc/apache2/mods-available/headers.load  /etc/apache2/mods-enabled/headers.load

Restart Apache2:

Also run with sudo

/etc/init.d/apache2 restart