Blank page when trying to add new content

Question

I've to cope with something very odd: I can't add any type of content anymore. For example: when I go to Content > Add content > Article, I navigate correctly to the 'add acticle' page, but I simply get a white screen. I didn't have this before and I can't remember installing new modules. I already tried to flush all caches, but that didn't solve anything. Even when I add

<?php
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
?>

to my index.php or settings.php file, no erros are displayed. Please help!

Edit 1: going to the Configuration screen doesn't result in a WSOD. There's only a white screen when I try to add content.

Edit 2: I discovered that I also get a WSOD when I try to edit already existing content. Please help me! I really can't do anything anymore!