customise site + customise link in appearance menu doesn’t work

Hi. The customise site button and the customise link in the appearance drop down menu in my dashboard doesn’t work. I get this message: The page isn’t redirecting properly. Firefox has detected that the server redirecting the request for this address in a way that will never complete. This problem can sometimes be caused by disabling or refusing to accept cookies.
I have tried deactivating/disabling all plug ins but this doesn’t seem to have done anything.
Any advice gratefully received.
Thanks