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Installing official cloudflare plugin on a new WP6 through Plugins - search.
The end result a blank page.
It was caused by .htaccess having require -lines. Wordpress installed through apt has sample apache config entries under /usr/share/doc which do not contain AllowOverride AuthConfig
Fix is to edit /etc/apache2/sites-available/yoursite ,
<Directory /usr/share/wordpress>
Options FollowSymLinks
AllowOverride Limit Options FileInfo
to
<Directory /usr/share/wordpress>
Options FollowSymLinks
AllowOverride Limit Options FileInfo AuthConfig
and apachectl graceful