Solution:
I had the same problem, website works fine, but access to the dashboard / wp-admin is just an empty page.
The problem in my case was that one or more of the plugins was using an outdated version of the https://github.com/rmccue/Requests library. WordPress new version 4.6 adds this library to the default install, but it seems that older versions in the plugin folder are preferred. So what you need to do is to find the plugin and de-activate it, or update the requests library in all other places yourself.
If you have shell access to the wordpress install, you can run ‘find . -iname requests’ in your wordpress plugin directory. Look out for all folders that are named rmccue/Requests or similar. Then either move that plugin away to remove it, or update the requests library directly by getting a new version from the above github link.
In my case the culprit was the jekyll-export plugin.