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 need root permissions seems not to open. As said, I could probably also open these using terminal and "sudo", but I don't know the commands.
Anyway, for some reason I yesterday also got Virtual Machine Manager working as remote desktop (before this wasn't working that's why I was trying to use VNC.) So I don't really need vnc working anymore. Would be interesting to know why it doesn't work properly though.