We are seeing something occasionally and randomly in our Sandbox installation, too. It can manifest right at the top level - no permission to view the main index page. It may stay that way despite a refresh, and then later, randomly, (and without any change to permissions) will right itself. I've seen it myself (an Admin), as my Alter Ego test a/c (a Moderator); and my colleagues have reported similar refusals. I cannot recreate it reliably; I'm just fairly confident it will occur again, but don't know when nor in what circumstances.