I ran into the same problem.
I can access the dashboards of my main site and of the subsites I created and a newly created subsite. All these sites are just “sub domains” without a registered domain name.
I cannot access the dashboard of the subsite that does have a domain name.
Of course, the first thing I did to solve the problem was to disable all the plugins and to activate the standard twentyfifteen theme.
I reinstalled WP 4.3.1 manually.
As I could not find a solution nowhere on the web, I started debugging WordPress. I found a work around:
In the wp-setttings.php file, I disabled the very last command in the file: “do_action( ‘wp_loaded’ );”. Now I can access the Dashboard of all subsites.
As I do not know WordPress on the inside, I have to figure out what the “do_action( ‘wp_loaded’ )” exactly does. As far as I understand, it is an API to which we can add our own code or from which we can remove existing code. I did not figure out what code is in the ‘wp_loaded’ action. I suppose something in there is causing the problem.
Maybe someone reading this reply could answer that?
The next few days I do not have time to dig deeper.
