LordDunvegan on "PHP Fatal Error wp_rewrite_rules()"

I have seen other people mention this, but cannot match the circumstances. I am not a PHP programmer, but just noticed I am generating enormous error logs.
All plug-in are up to date and am using BuddyBoss theme.

Full error is
PHP Fatal error: Call to a member function wp_rewrite_rules() on a non-object in /var/www/html/wp-includes/rewrite.php on line 510, referer: https://explore.scimednet.org/wp-admin/post.php?post=7$

This looks like core code and is happening several times each minute.

Can anyone help please?

Thanks

Richard

disable all plugins and change your theme to twentysixteen. If the problem goes away, re-enable things one at a time until you see it retu. Then, we have something to talk about.

Thank you. Will try this on my test server. I cannot do it on the live one.
Richard