1. This is a possible corruption of the pop3uidl file. Close your internet connection and all programs. Search for the file pop3uidl and delete it. Reconnect and it should work.
2. Repeat step 1 with a reboot before you check your mail again.
3. You may have a corrupt or a very large file attachment on the mailserver. Large file attachments, mail that are missing headers, or mail that do not conform to the mail RFC's can cause this.