Mcrypt is installed, but error implies it's not.

have you tried to use mycrypt as standalone php. maybe you get there also an error?

PHP Code:

echo trim(base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, 'your-salt_bla123', 'the secret message', MCRYPT_MODE_ECB, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB), MCRYPT_RAND))));