error with multicheckbox in IE 8

First of all excuse for my english because I'm from Spain and thanks for your help in advance.
I have created a registration form with multiple fields with CB an when I try to register an user in mozilla firefox all it's ok, but when I try to register using IE 8, it's impossible because it gives me an error in multiple checkbox fields, it seems like if this fields are empty, and like they are obligatory, I can't register my own. I don't have any idea about how to fix this.
Help me please. Yo can find the registration form in
casaprivee.com/unete/registrate .
Thanks for your help.

it's impossible because it gives me an error in multiple checkbox fields, it seems like if this fields are empty, and like they are obligatory

You have the fields configured as Required, you can change this within Field Management otherwise you'll be required to select at least 1 of the values to register.

Thanks, I know I have it like required, and I want it like that, the problem it's that I select at least one value and in IE for the multichecbox, it says that the fields are empty, when the true it's that I have select one or more value, and I don't know why?

Was not able to duplicate locally. Was able to confirm on YOUR site however that this is the case. Seams there is some conflicting JS likely from your Template, a Module, a Joomla System Plugin, or a CB Plugin that is NOT compatible with IE. This would cause a fatal JS error and prevent any further JS from executing (causing the validation check to mess up).

Viewing source there appears to be 3rd party JS loaded that may not work in IE. The following extensions seam to be loading unfamiliar JS and I recommend testing disabling them 1 by 1 and confirming if resolved or not.

Joomla Plugins:
avreload - Content Plugin
rokbox - System Plugin

Joomla Components:
com_gantry

Thanks for your answers.
I've tried and when I change my template to another predeterminate of joomla all it's ok and the error dissapears, so I suppose it's an incompability between my template (quantive from rocketheme), the multiplecheckbox and IE.
Unfortunately for me, I don't know nothing about JS code, so it's impossible for me to fix this problem... May be you could help me? If not, I will test with drop down multi-select, but I don't like this very much because you have to push "Ctrl" to select more than one option... it's possible to change this?
Thanks a lot!!

when I change my template to another predeterminate of joomla all it's ok

This means your template has JS that is not compatible with IE. You'll need to contact the developer and have them fix this or use a different template. What's happening is the templates JS is having a Fatal error, which is preventing any further JS (CBs) from working.

May be you could help me?

No, we don't fix 3rd party extensions; sorry. You'll need to contact the developer.

it's possible to change this?

No, as far as I am aware there's no easy way to change from drop-downs without doing a lot of core edits which includes changing the JS to work with the new drop-downs. This would not be an acceptable solution as your template would continue to not work in IE.