This is still happening. With no particular hint as to why or how to fix it =/
It appears to be related to certain PM chains - when a reply chain become bugged for whatever reason causes it, you are unable to submit replies to it, though it seems to be fixed by restarting the computer or phone sometimes. However, starting an entirely new PM chain will circumvent it. This is troublesome, however, when you are relying on reply chain logs to reference old data from previous PMs.