one item that I've had scramble with past upgrades are the redirectURIs for the arcgisonline appID. I have had to check to make sure that the DNS uri, machine name uri, our local domain uri and the the *.arcgis.com uri are all in place after upgrade:
usually just having the arcgisonline appID configured correctly handles all the other appIDs that you could register, but maybe not . . . just something to check.