Suddenly - I don't know since when - I can't edit menu item types anymore. When I click the select button, a pop up appears, but it stays empty. It's the same for new menu's and menu items.
Does it seem familiar to anyone? I already searched a lot, but I can't find other people with the same problem.
It seems like the javascript is not loading.
I'm running Joomla 3.4.3
If JavaScript was broken, I guess you would not see the modal at all.
Increase the error level in Joomla configuration (System -> Global Configuration -> Server -> Error Reporting, and maybe in the modal page you will see a clearer error message).