I just installed 5.6 and got this exact same error. The JQuery Migrate plugin didn’t make a difference, sadly. I opted to restore the backup I made before the update, only to then have WP force me to re-install 5.6 before letting me go back to the site.
Now I just have this message: “There has been a critical error on this website. Please check your site admin email inbox for instructions.”
I haven’t received any email. I can’t even access the dashboard anymore.
