That's very odd. Rapidshare checks your IP to impose download limits, so as long as you've not downloaded anything very recently it should work fine. There shouldn't be any limit on how many times something can be downloaded by multiple users, so it can't be that - I've not had any problems downloading stuff myself. You're not using PCs on a shared network connection are you?
The advantage I get with Rapidshare is that I get to see how many downloads each file has, which is very useful, but if the problem persists I can probably shift the calendar to my web space as the downloads have most likely peaked by now anyway.
Thanks for the offer of wibble storage esteban. I'll keep it in mind for the future (I'm still evaluating the downloads at the moment to see if I can do the same as above and move it to my own webspace).