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 (you may have to search hidden files). 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.