I just tried sending Niko a one letter message and it worked. I have been using the PM system here for years now and have never seen this error. I normally use Firefox on Linux and do have quite a fast connection although on rare occasions I do use FF on W7 and its not happened on that either. The cookie thing mentioned in Bashy’s post could perhaps play a part but again I have never cleared mine out deliberately and usually my FF profile stays across system rebuilds as I use the same /home directory on Linux and save it on Windows prior to updating.
Stuart