Suhosin is a security extension that is sometimes added to PHP.
One of it's settings restricts the length of individual GET (url) parameters to 512 characters.
Sagepay Form relies on a GET parameter called 'crypt' that can be much longer than 512 characters.
To fix the problem you will need to add an entry to your php.ini file.
suhosin.get.max_value_length = 2048
If you don't have access to your php.ini, you will need to speak to your hosting provider.