I thought this was only an issue on the iPad, but it happened on my PC as well, both using Chrome:
Someone sends you a PM.
You come here and log in.
A dialog box tells you have a PM, so you click to go to it.
At that point I cannot reply to the message. If I exit the browser and come back in, there is no pop-up dialog box, and if you go into that PM, all is well.