Thanks for pointing me in the right direction! I had this issue after upgrading PHP to latest 5.6.(blank page 500 error after logging into Manager)
I disabled widgets in PHPMyAdmin and found that the "Upgrade MODX" widget is what was breaking it.
This worked for all my sites on this particular server.
Thanks again!
Did you figure out why the UpgradeMODX widget was causing trouble? I haven't been able to reproduce your problem.
