Today all the author pages return 404 when clicked.

They used to work until recently. I am not sure what has gone wrong. What could be the cause of this problem? 

I noticed it happened when I deactivated the plugin "Rank Math SEO". The problem is fixed when I enable it.
How can I return things to the default settings? 

Found a solution to the bug here: https://support.rankmath.com/ticket/author-page-broken/

You may close this ticket. 

Thank you.