Pluggable Line 866

traced the issue to a 302 redirect that was generated at login so just went to a non-broke site and copied the url and changed the domain to get past that redirect. To do it use this url with your settings

https://www.mydomain.com/wp-login.php?redirect_to=http%3A%2F%2Fwww.mydomain.com%2Fwp-admin%2F&reauth=1

You’ll be able to login here. In my case the NextGen plugin was in need of a critical update. I updated it and problem went away

This one worked very well for me. I was able to log into my previously inaccessible backend.

In my case, it was the misbehaving staff-directory plugin that was causing the login error.

Thanks a lot to JerryOdom.