I've got a 304 on my Scout but I have'nt encountered your problem with mine so far. Could be because I don't use the guard. Wish I could be of help to you.
the more I think about it the more it looks like a manufacturing fault. I suppose the cover needs to be removeable in case the stylus assembly has to be changed but obviously it shouldn't be loose. Removing the stylus guard does exert some pressure on the cover but it should be able to withstand that. I wonder what Denon will have to say about it - I've heard Audio Technica are very helpful so let's hope Denon are as well.