I ran into the same problem. Something about the update hosed my wordpress site. I had to go through and manually reinstall wordpress 4.9.6. There are lots of different instructions regarding the process on the net, but it boils down to this:
– stop web server
– download latest wordpress version
– backup existing site
– delete wp-admin and wp-include folders on existing site
– copy over same folders from downloaded version
– copy over wp-content from downloaded version to wp-content folder of existing site (do not delete its contents)
– make any other modifications to wp-admin/wp-include that you had before.
– start server
And then you should be able to get back in.
Granted, this is a very laborious and error-prone process, so caution should be taken. Also I in no way guarantee or support such a method, although I did successfully restore my site.
