I have talked to Shalidor and he opened the portal to Shivering Isles. I did not use the portal immediately. When I returned later, the portal was gone, as well as Shalidor. The journal still says "talk to Shalidor", but he is not in the mages guild anymore. Sometimes I can see a circle on the floor where the portal should be, and there is a radar marker for Shalidor, but he is invisible and I cannot initiate a dialog.