The MCrypt Actions plugin, part of the Serial Number Validation package, was updated to version 1.0.4.1 today.
An error was found in the way the padding was handled in the 256-bit AES PHP-compatible MCrypt action. This error could result in an incomplete decrypted string, if the ciphertext would meet certain requirements.
All users of previous versions are urged to update to the latest release to avoid problems. It is not required to replace the PHP pages on your web server, but you may want to install them locally, then upload the serial_verify.php and offline.php files manually (using the file names you use in your environment).