I can report it for you if you like, because that should only come up when you have rated that thread.
Have you tried clearing cookies etc (I have no idea what this does, but people often suggest it )
I suspect that for some reason the thread in question thinks ,you have rated it (well duh, ) Unfortunately the only person who is likely to be able to take a rational stab at a possible answer is on holiday for two weeks.
Have you tried on another thread that you have never looked at before?
Fiona - you are quite right: I logged out of my moderator account and tried a test member account and sure enough, when I try to rate this thread I get the "you have already rated this thread " message.
I will report it. It may be a system setting but I will report it.