There are two hiccups regarding your private messages that seem to be fairly common here.
One, even though your inbox shows as being empty, the status will say (like mine did) "0 messages. You have 379 message of 100 allowed".
Until you fix that, you can neither send nor receive private messages. I have tried to send pm's to members who have not discovered that reality yet and get a response that their message folder is full.
Check your inbox. Look at the amount that it says you have stored. You will need to work in the pm box until that is emptied as for some reason the system carried over every message you have ever sent and has stored it in such a manner that the system thinks your email box is over-loaded.