Sometimes, if you see a blank page after an operation, it might be a hidden warning. You can enable it for debug purposes by changing a special value

1. on the FTP, open /config/
2. Line 28 
define('_PS_MODE_DEV_', false); 
 change to 

define('_PS_MODE_DEV_', true);
 than you might see a php warning instead a blank page, that might help you to solve the issue