When I click on "Add new item" in my user-menu I get a pop-up view showing the new item form perfectly.
But when I click on "Close" the pop-up view loads an empty page.

When I edit some text and click on save, the pop-up view remains showing the item form with the message "Item saved"

When I just save a empty form the pop-up view shows an error page saying "404 - Item not found"

Item not found

Call stack
# Function Location
1 JSite->dispatch() /var/www/index.php:43
2 JComponentHelper::renderComponent() /var/www/includes/application.php:191
3 require_once() /var/www/libraries/joomla/application/component/helper.php:156
4 JController->execute() /var/www/components/com_k2/k2.php:52
5 K2ControllerItem->display() /var/www/libraries/joomla/application/component/controller.php:684
6 JController->display() /var/www/components/com_k2/controllers/item.php:60
7 K2ViewItem->display() /var/www/libraries/joomla/application/component/controller.php:650
8 JError::raiseError() /var/www/components/com_k2/views/item/view.html.php:44
9 JError::raise() /var/www/libraries/joomla/error/error.php:250

I have an other account (Super User, Site Owner) accont that does work. The pop-up closes perfectly.
But when I make this user Super User and Site Owner, it does not help. So I don't think this has to do with rights.

Also I don't have any template-overrides active.

What can this be? Hopefully someone can help me fix this problem on my live Intranet site. :)

Please does someone know what this could be?

I found it! I have the module mod_simplefileuploadv1.3 installed on my site. This module seems to kill some functionality to close the pop-up when you click on the close button.

When you have access to this module so you can upload files it works fine. But when you are not allowed to upload files with this module the pop-up function breaks.