This seems more like an Apache error, but I'm not finding much info when I search for it so maybe you can help. As soon as I save the configuration for the plugin I get a 500 Internal Server Error from Apache. Looking into the Apache error log I see this: .htaccess: RewriteRule: unknown flag 'END', referer: http://dellaoctober.devel/backend/system/settings/update/offline/responsiveimages/config

All of my research shows that END is a valid flag so I'm quite confused about where the problem is.

Make sure you are using the right Apache version: https://github.com/OFFLINE-GmbH/oc-responsive-images-plugin/issues/53

You can also manually patch the added .htaccess rule to work with your server version.

Apache version was wrong. I upgraded to Apache 2.4.46 and the error went away. The plugin is working now. Thanks for your help.`