(ERROR) Custom fields (Cart Variant) - Price Update

[hugopt]

Hello,

I'm using vm 2.0 and joomla 1.7.3

When I add custom fields (cart variant) to a product like this:
Product: PC
HDD:   
- 1TB 50€
- 2TB 100€

In the product page when I choose the HDD configuration the price automatic change.

But now there's a problem with the SEO and multi-language system of Joomla.
If I deactivate (add sufix in url) the option (Remove default language prefix) in the language plugin (System - Language Filter) the price is no longer update when choose the HDD configuration.

The price update only work with this option enable (remove sufix), but like this the Joomla multi-language doesn't work.

What can we do?

Thank you.