Thanks for your quick reply. I maybe wasn't very clear in my question. Typing "sudo synaptic" works fine. The problem is, it is not only synaptic that isn't working. eg I also can't open "system settings", "system monitor", "preferences" anything that could ne...

Hi wsimm, Did you manage to fix this problem? I'm encountering exactly the same. I can open Synaptic via Terminal with "sudo synaptic", that way it opens and works fine. My main problem however is that also a lot of other GUI's don't open (Software, System settings, ...) I assume they also...