Thanks for the admin credentials. Unfortunately, it's not possible for me to narrow things down. When setting Error Reporting to "Maximum" in the Global Configuration area, that should display the actual error message on the blank page. However, something is preventing those errors from displaying. It's either your host has disabled error reporting altogether or another extension is preventing them from displaying (which is bad behavior).
You can tell that something is preventing them from the Site -> System Information area. If you search for the "error_reporting" value, it shows:
error_reporting 0 30711
The 30711 setting is the correct value for the "Master" value column that Joomla sets. The "Local" column is 0 though, which means it's been overwritten to be off.
You'll need to contact your hosting provider to see if you can obtain the PHP Error Logs (not the Apache logs). In those, there should be a Fatal Error that will explain what is happening in the admin area.
I'm not sure what's going on right now as I've never seen the problem you're having before. The error message should narrow things down very quickly.
Sorry for the trouble, but we'll get you going,
Alex