Somehow I hadn’t paid attention to that one because it was missing the key word “Primary script unknown”, and the post with these keywords https://help.nextcloud.com/t/primary-script-unknown-after-updating-nginx-php-fpm-configuration/65020/3 didn’t help me,
But yes, the thread you quoted would have saved me some time by letting me look at the memecache configuration.
However, I found a quick fix from the thread https://help.nextcloud.com/t/this-version-of-nextcloud-is-not-compatible-with-php-7-3/65332/4
I don’t know why I can’t make apcu work with php7.2, but hopefully it doesn’t matter.