Hi Acrono,
I saw the same issue except my entire site is down, admin too. You can rollback on a WP version easily by going to
https://www.ads-software.com/download/release-archive/
Select the version you want, unzip it and then using an FTP app like FileZilla, overwrite the wp-admin and wp-includes folder and also all the root files (except wp-config) with the newly unzipped version files.
This will get you back to the version you selected without affecting your themes, plugins etc.
I’ve done this loads of times through the years when I make a mess of things.
Hope this helps,
GB