How to enable error logging / debug white blank screens.

Edit the file includes/config.php (via FTP or a cPanel file manager). Near the bottom of the file is this block of code:

ini_set("display_errors",false);
ini_set("error_reporting",0);

Change it to this:

ini_set("display_errors",true);
ini_set("error_reporting",E_ALL);

Then repeat the previous action in UCM and see if any errors display on the screen.