Admin Login Redirect – Can't login after upgrading to WP 4

After upgrading to WP 4, we are unable to login to the back end of our site at /wp-admin. I’m at my wits end after trying almost I’ve ever seen suggested:

– logging in from different machines (same issue)
– disabling the themes folder
– disabling the plugins folder
– restting the site URI and Home in wp-config.php (using www. and without)
– reuploading a fresh version of the entire wp-admin folder
– deleting all .htaccess files
– clearing cookies/cache all along the way

I have tried turning on debugging, thinking that might shed some light on it. But I cannot get WP to create a debug log file.

One additional note: Something also seems to have happened with our clean urls, and though our homepage loads at https://www.hexanine.com/zeroside, no other interior page or link will load. They all redirect back to hexanine.com, and none of the images load. Ugh.

Has anyone else had this issue? Can anyone please offer some advice?