Steps navigation and "You are not allowed to access this UR"

Hello,

After reading about this same topic in lots of posts and not be able to fix the problem i am posting a new one.

My problem:
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i have a multipage form (3 steps).
Steps navigation is enabled.
The third step works as a confirmation page because i read (and experimented) the problems using the multipage and confirmations plugins together
The back button in the third step is "<a href="https://forms.cat.org.uk/index.php?option=com_chronocontact&task=send&chronoformname=CAT_Members_Renew_Membership_Form&cfformstep=2"><button type="button">Back</button></a>"
When i click this abck button i get the "You are not allowed to access this URL"
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Things i have read in different posts and tried but didn´t work:
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Setting on and off "Republish fields if error occured" in all the forms
Check the antispam is off
Set the "checkToken" in all forms to OFF/ON
Turn On/OFF the cache in the global configuration (Joomla)
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Things that my affect to the problem:
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The form is used to renew membership. In step 1, the user introduces some data that is used in the step 2 to load his/her membership information
In step 2 we load this data using the very famous script

<?php
if(!$mainframe->isSite()){return;}
$formname =& JRequest::getString('chronoformname','','post');
$session =& JFactory::getSession();
$posted = $session->get('chrono_formpages_data_'.$formname,array(),md5('chrono'));
?>

Crhonoforms version: v.3.2 (validated)
Joomla 1.5.22
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In case anybody wants to test the form:
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Member nº: 00000
email: [email]test@test.test[/email]
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Thanks very much for your help
carlos