If you are using IE6 in particular, the following will be necessary to clear things before you sign in.
This happens to my notebook computer after I resume.
Tools >> Internet Options >> Delete Cookies >> Delete Files >> OK should work.
I now do this automatically after a resume if I want to use ebay as it seems to block me 80% of the time.
If I want to delete any items from My favourites section, it says sign in followed by "No such user" I then have to go through the above process and it will delete items.
Don't ask, haven't a clue!
I hope that helps, though I have never had a similar problem signing in to AS, cache full or not!