How to deal with the "Payment failed at the payment server" error? I'm trying to get my website integrated with SagePay.
Everything works well untill the payment is made.
Once a client makes payment he/she gets the following error message:

Notice: Undefined index: crypt in commerce_sagepay_form_redirect_form_validate() (line 55 of /sites/all/modules/commerce_sagepay/includes/commerce_sagepay_form.inc).

Payment failed at the payment server. Please review your information and try again.


And I get the following info in the logs:

Response could not be converted to tokens


Despite these errors the payment was taken, but the order remains in the shopping cart with the status Checkout: Review

I'm using Commerce SagePay 7.x-1.0 and the integration option is Form.
I had a similar problem with the token conversion, be sure you're using the -dev version (2014-May-23 or later) of Commerce Sagepay before trying anything else. 

Note: probably related to: https://drupal.org/node/2115203