Sorry that you're having that issue, there are reports that this error is related to JetPack or Custom 404 extensions if you have those plugins try to deactivate it and see if that resolves the issue. If it does, please confirm that you are fully updated? (Theme, Cornerstone, Extensions, and Plugins)
You can find the latest version numbers here. Then you can compare them to what's installed on your site.
If you find anything to be out of date, you can review our update guide.
Another, reports that were related to this is the PHP version 7.2. so if the solution (updating) above did not work, please contact your hosting support and ask how you can downgrade the PHP version to 7.0 or 7.1. This should be on your cPanel.
Let us know how it goes,