The forbidden message can be caused by a corrupt browser cache or cookie. Have you tried a different browser? It can also be caused if the account has been locked by VM, but that is usually preceded by an unavailable message.
You mention it is an old account.
Does anything in this thread correspond to your situation?
I have tried another browser etc and the usual things.i think it may be locked as I got a letter some time back saying my account may have been hacked so I had to password reset which I did, but now I've realised I cannot enter my mail