Try editing your wp-config.php file ( it is located on the root folder of your installation ) and right before the lines that say:

/* That's all, stop editing! Happy blogging. */

add this

define('CONCATENATE_SCRIPTS', false);

This will tell WordPress to load each script on it’s own instead of combining them. This might be caused due to some faulty or outdated plugin .js that overrides something else when it is concatenated.