I have similar thing, turns out it’s malware. There is some extra files and the malware disables updates by editing theme files. Weird thing is that I have almost no plugins and according to log same date as the new files was creates there were tons of request to xmlrpc.php file. Which is even more weird because I have xmlrpc_enabled set to false.
I recommend restoring your site from backup, and by restoring I mean also deleting current folder (if you don’t want to find extra files manually) and put entire one from backup.
