Gmail.com: "[SYS/TEMP] Due to a temporary system problem, this mailbox will seem empty."

Problem: I have over 100 emails sitting in a Gmail account, which can't be downloaded via the POP interface, even though POP is enabled for all messages. I created the account in January 2006. Meanwhile another account I created last year works fine. The unusable account gives my mail client (Outlook Express or jwSpamSpy as its anti-spam prioxy) the following error message:

[SYS/TEMP] Due to a temporary system problem, this mailbox will seem empty.

Solution: So far I haven't found one. Let me know if you find one!