Perhaps related to the other errors I just reported, I also see a few of these in the dovecot-2.0.3 logs: lda(pid 8212 user user3): Error: Opening INBOX failed: Mailbox doesn't exist: INBOX lda(pid 8212 user user3): msgid=<201009240906510365.1gj1fpfhrhhm9@mail>: save failed to INBOX: Mailbox doesn't exist: INBOX lda(pid 8212 user user3): Error: BUG: Saving failed to unknown storage
and some clients report receiving this IMAP error: tag NO Mailbox doesn't exist: INBOX
although in general mail delivery and IMAP work, and these users' Maildir++ directories do exist and look OK. This error did not occur for the user1 or user2 in the other report.
And here's a third kind of error: imap(pid 5722 user user4): Error: stat(/path/to/mail/user4/tmp/ΝΌ) failed: No such file or directory imap(pid 5722 user user4): Error: Internal quota calculation error
That path looks like a memory munge.