I had been having a similar issue, and this is how it was resolved.

PROBLEM: Logging in as Admin produced a blank page.

SOLUTION:

Log-In as Admin (get blank page), then modify url to admin section:
(http://yoursite.com/admin.php)

Once in, turn on PHPDebug from Preferences-->System--General Settings

Save.

Then, open your site again. An error message should follow. In my particular case, the error was:

Fatal error: Call to undefined function: error_die() in /homepages/18/d128327878/htdocs/mastery/modules/uskolaxgallery/blocks/gallery.php on line 162

From this point, still in the Admin area, I went into "Blocks", deleted the faulty block that was causing the error.

All is well now.

Hope this helps any other souls that may encounter the same.

Cheers