Il problema veniva da un override del template, uso Creatrix.
Quando ho provato a cambiarlo, come mi aveva detto Luketto, l'ho cambiato solo per la pagina di registrazione e, probabilmente, rimaneva l'override.
Per chi avesse lo stesso problema con lo stesso template (penso sia difficile)
Il file in questione sta su template>jm_creatrix>html>com_users>registration>default.php
La fine del file va modificata da così
<?php echo $field->label; ?>
<?php if (!$field->required && $field->type!='Spacer'): ?>
<span class="optional"><?php echo JText::_('COM_USERS_OPTIONAL'); ?></span>
<?php endif; ?>
<?php echo ($field->type!='Spacer') ? $field->input : "&#160;"; ?>
</div>
<?php endif;?>
<?php endforeach;?>
</div>
<div id="jm-registration-bt" class="clearfix">
<button type="submit" class="validate left btn btn-trans-gray"><?php echo JText::_('JREGISTER');?></button>
<a class="jm-btn left btn btn-trans-gray" href="<?php echo JRoute::_('');?>" title="<?php echo JText::_('JCANCEL');?>"><?php echo JText::_('JCANCEL');?></a>
<input type="hidden" name="option" value="com_users" />
<input type="hidden" name="task" value="registration.register" />
<?php echo JHtml::_('form.token');?>
</div>
</div>
</fieldset>
<?php endif;?>
<?php endforeach;?>
</form>
</div>
A così:
<?php echo $field->label; ?>
<?php if (!$field->required && $field->type!='Spacer'): ?>
<span class="optional"><?php echo JText::_('COM_USERS_OPTIONAL'); ?></span>
<?php endif; ?>
<?php echo ($field->type!='Spacer') ? $field->input : "&#160;"; ?>
</div>
<?php endif;?>
<?php endforeach;?>
</div>
</div>
</fieldset>
<?php endif;?>
<?php endforeach;?>
<div id="jm-registration-bt" class="clearfix">
<button type="submit" class="validate left btn btn-trans-gray"><?php echo JText::_('JREGISTER');?></button>
<a class="jm-btn left btn btn-trans-gray" href="<?php echo JRoute::_('');?>" title="<?php echo JText::_('JCANCEL');?>"><?php echo JText::_('JCANCEL');?></a>
<input type="hidden" name="option" value="com_users" />
<input type="hidden" name="task" value="registration.register" />
<?php echo JHtml::_('form.token');?>
</div>
</form>
</div>