Little Paddy I experienced the same problem on more than one occasion. I have had four usernames suddenly crap out on me and refuse to allow me priveleges. Now and then, I try my previous username and if I persist I can eventually post but this "itchybum" ALWAYS gets me in to all services.
For the previous to work I had to delete cookies and temp internet files then log in and even then I had to do it several times and fail before it actually got me to the POST window. Then after I finish typing and hit SUBMIT (or whatever) it often went back to the log-in window and I had to start again. Very annoying.
What's more annoying is there was no explanation from pprune except that it must be a PC problem. Strange since itchybum gets in every time with the SAME PC but I could not get this point across and they eventually got tired of offering advice.
ask users who have the same Username for nearly seven years why they don't have a problem. There is no other reason why a Username becomes 'unaccessible'.
Well obviously there must be another reason since it sounds, L.Paddy, as though your problem is not unique.