On a web site running Apache, i got following error in log file
[Fri Jan 19 06:43:41.146643 2018] [core:alert] [pid 3536:tid 139697024132864] [client 188.8.131.52:50559] /home/site.com/public_html/.htaccess: Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration, referer: https://site.com/
This was Ubuntu server with Apache.
Problem is due to Apache Headers module not enabled. Fixed by running
systemctl restart apache2