I’m not sure if this will help but today I started getting all 404 errors except on my home page. The blog posts, member pages, forum pages, etc all 404 errors. First I deactivated all my plugins and then checked that, still 404 errors so I reactivated them.
Then I changed the permalink structure I had set back to default. This fixed it. I then switched the permalink structure back again to my preference and what all my exiting links are (previous setting) and it is now working fine.
I don’t know why this happen but maybe doing that will help you as well. Good luck
