On app/config/app.php just change:'cipher' => MCRYPT_RIJNDAEL_256,
Run php artisan key:generate on your command line to refresh your application key.
php artisan key:generate
Then clear your cache and sessions and try again.
I had this same problem today, and it was because of how Laravel 4.2 uses the new encryption method.
Make sure you run php 7.0, and add this string "MCRYPT_RIJNDAEL_256" (without the quotes) to the cipher in app.php. after generate a new key (php artisan key:generate) and add the new key.This works for me.