Check the reporting services logs then - you have some type of permissions errors going on which should be in the log. If it used to work then most likely something has changed. Are you doing any of this from being logged onto the server itself or are you doing this from your PC?
And you need to make sure you right click IE and select Run As Administrator...it's different than logging on as Administrator and running IE or being in the local admins group and opening IE.

Sue