wrong type argument

[Mahdi Osman]

Sorry again for my question.
I am using emacs 21.3 and I have the following problem.
Sending mail using gnus gives me "wrong type argument: stringp, nil" and
the cursor jumps to "From:" which is me. My mail is rejected to sent to
 draft.
All my incoming mails are marked "O". Even after unmarking them, when I
restart my emacs and gnus, mails are still marked "O".
The following is the trace I found:
...........
+OK AVG POP3 Proxy Server <23946.1114410300@mp006> 7.0.307/7.0.308 [266.10.2]
+OK May I have your password, please?
+OK mailbox has 0 messages (0 octets)
+OK mailbox has 0 messages (0 octets)
.
+OK bye
Process POP exited abnormally with code 256
.........................
Finally I can not exit gnus by using "q". It hangs up showing
 "wrong type argument: stringp, nil" in the mini buffer.
I hope somebody can give me a hint on how to overcome these problems.
Thanks a lot
-- 
Mahdi Osman (PhD)