It might be that, VS... now I've been able to send one. I'll keep an eye on this anyway.
Btw, I'm not sure how useful is a per-hour-limitation... I tend to answer all of my PMs (if any) in a row, despite there aren't a lot of them most of the times.
Obviously such a limitation would prevent a potential spammer to send a bunch of PMs at a time, but... well, that.