Can't validate Greek VAT

Whenever a Customer from Greece tries to enter the EU VAT number, this message is returned:

Your VAT number can not be used with the country you selected. Please select the right country ( CODE: EL) or make sure your VAT starts with GR

I tried with a valid* VAT number in all possible ways:
- only digits: returns the message above
- EL before the digits: returns the message above
- with GR before the digits: returns this error message "The VAT number you entered is not valid" and changes the input field as ELGRdigits.

(*) verified on Vies

-- HikaShop version -- : HikaShop Essential: 2.2.1
-- Joomla version -- : 1.5.26
-- PHP version -- : 5.3.3
-- Chrome, Firefox

Thanks.
The situation improved.

now the situation is:
- only digits: returns the message (*)
- EL before the digits: returns the message (*), and displays it into the modal window
- with GR before the digits: it works

* Your VAT number can not be used with the country you selected. Please select the right country ( CODE: EL) or make sure your VAT starts with GR