Wifi problems after 4.4 KitKat update
I found out what the problem was: on every single device there were 'battery saver' apps installed. The devices are in my workplace, so I'm not the only one who uses them and someone must have installed those apps. 
I didn't care for them until now and they never did any harm, but they somehow must've interferred with the new 4.4 Android. After uninstalling them everything is fine on all devices :)