Thanks for the info, if this is the issue and you can’t enable mod_headers on your server, or get your hosting company to enable, you can change these parts of the htaccess:
1) Change:
<filesMatch "\.(html|gz)$">
Header set Vary "Accept-Encoding, Cookie"
Header set Cache-Control "max-age=5, must-revalidate"
</filesMatch>
To:
<IfModule mod_headers.c>
<filesMatch "\.(html|gz)$">
Header set Vary "Accept-Encoding, Cookie"
Header set Cache-Control "max-age=5, must-revalidate"
</filesMatch>
</IfModule>
2) Change:
Header unset Last-Modified
To:
<IfModule mod_headers.c>
Header unset Last-Modified
</IfModule>