It is happening to me for about a few months, I dont send PM very often and I noticed it happens almost always when I try to send the first PM. Then Im blocked, I try again, Im blocked again and then third or fourth time it finally works and then until that session ends (ie. until I for example dont look at the forum for several hours) it works fine.
So I was sending a PM to Brian about 20 minutes ago and it happened. When I try now it works ok but I bet if I try tomorrow, the first time I try it will happen again