Hi,
this post helped, but the problem was this:
templates/flexible_lightmart/html/com_virtuemart/user/login.php
row 175:
<a class="flexibleButton flexibleModal" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&tmpl=component&flexible=guest&task=editaddresscart&addrtype=BT',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?>"><?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?></a>
This row has to modify to absolute url with your domain (not menu's item):
<a class="flexibleButton flexibleModal" href="<?php echo JRoute::_('http://yourdomain.com/index.php?option=com_virtuemart&view=user&tmpl=component&flexible=guest&task=editaddresscart&addrtype=BT',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?>"><?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?></a>
templates/flexible_lightmart/html/com_virtuemart/user/edit_adress.php
row 68:
<form method="post" id="userForm" target="_parent" name="userForm" class="form-validate <?php echo ($parameter == "guest")?'flexibleGuest':''?>" action="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user',$this->useXHTML,$this->useSSL) ?>"
This row has to modify to absolute url with your domain (not menu's item):
<form method="post" id="userForm" target="_parent" name="userForm" class="form-validate <?php echo ($parameter == "guest")?'flexibleGuest':''?>" action="<?php echo JRoute::_('http://yourdomain.com/index.php?option=com_virtuemart&view=user',$this->useXHTML,$this->useSSL) ?>" >
Regards.