I put this php-code into /html/com_content/category/blog_item.php
<?php if ($params->get('show_readmore') && $this->item->readmore) :
if ($params->get('access-view')) :
$link = JRoute::_(ContentHelperRoute::getArticleRoute($this->item->slug, $this->item->catid));
else :
$menu = JFactory::getApplication()->getMenu();
$active = $menu->getActive();
$itemId = $active->id;
$link1 = JRoute::_('index.php?option=com_users&view=login&Itemid=' . $itemId);
$returnURL = JRoute::_(ContentHelperRoute::getArticleRoute($this->item->slug, $this->item->catid));
$link = new JUri($link1);
$link->setVar('return', base64_encode($returnURL));
endif; ?>
<?php echo JLayoutHelper::render('joomla.content.readmore', array('item' => $this->item, 'params' => $params, 'link' => $link)); ?>
<?php endif; ?>
But I don't know if that is the right solution. Was something missing in the blog_item.php?