Sometimes you will get this error whenever your try to open Magento admin dashboard

“Mage registry key “_singleton/massemail/observer” already existsv”

Fix : You can solve this issue by clearing Magento cache.

Login to your server via SSH

cd /pathtomagento/shell

and run compiler.php

php -f shell/compiler.php disable
php -f shell/compiler.php clear
php -f shell/compiler.php compile