if you have a Wordpress website, congratulations. But: Check your website.

I have just seen something like I experienced. PHP/JavaScript warnings top and bottom across the screen. Website otherwise functional, until you fail to log-in and sort it!

The problem turned out to be an update of PHP to v7.2 and not updating to Wordpress 5.0.2.

Wordpress needs updating while you still have (or rolled-back to) PHP v7.1. If you have auto-updates on PHP and not Wordpress - let us hope it is possible to roll back PHP. The two are not linked and in totally different environs.

Then there is the issue of all the Addons, widgets, Plugins, and Themes that need updating, &/or finding the one that is the root cause of the conflict.

Best of luck.