Numerous admin pages not loading – reinstall/disabling plugins doesn't help

I’m getting this error:
http://screencast.com/t/qYltNLvnu

When trying to access the following admin pages:
/wp-admin/update-core.php
/wp-admin/themes.php
/wp-admin/plugins.php

I have tried several things to fix this based on other topics here:
1) Manual re-install of wordpress
2) Using a back-up from prior to 4.3 update
3) Disabled all plug-ins via phpMyAdmin
4) Tried disabling my theme (with complete failure)

I’m at a loss now. Any ideas on how to get these pages back to operation? My site loads and seems to operate normally despite that. I’m using Generate Press (premium) theme and thought that might be the problem but have found no compatibility issues listed on their forums.