Hm, I found more revealing errors after sending that message. These are scrubbed a little differently, to show two user names:
imap(pid 5720 user user1): Error: open(/path/to/mail/user2/tmp/1285336854.M157825P5720.my.mail.server) failed: No such file or directory imap(pid 5720 user user1): Error: Maildir: Symlink destination doesn't exist: /path/to/mail/user2/cur/1285336100.M879758P1166.my.mail.server,S=41045,W=41755:2,FS imap(pid 5720 user user1): Error: open(/path/to/mail/user2/tmp/1285335855.M242128P1165.my.mail.server,S=41045,W=41755) failed: No such file or directory imap(pid 5720 user user1): Error: Maildir: Symlink destination doesn't exist: /path/to/mail/user2/cur/1285336337.M656599P1166.my.mail.server,S=41045,W=41755:2,FST imap(pid 5720 user user1): Error: open(/path/to/mail/user2/tmp/1285335855.M242128P1165.my.mail.server,S=41045,W=41755) failed: No such file or directory imap(pid 5720 user user1): Error: Maildir: Symlink destination doesn't exist: /path/to/mail/user2/cur/1285336337.M656599P1166.my.mail.server,S=41045,W=41755:2,FST imap(pid 5720 user user1): Error: open(/path/to/mail/user2/tmp/1285335855.M242128P1165.my.mail.server,S=41045,W=41755) failed: No such file or directory imap(pid 5721 user user2): Error: Maildir: Found unwanted directory /path/to/mail/user2/cur/:2,FST, but rmdir() failed: Directory not empty imap(pid 5721 user user2): Disconnected: Internal error occurred. Refer to server log for more information. [2010-09-24 09:07:23] bytes=1268/173293
Note that user1 != user2. Seems like this might be caused by having: service imap { client_limit = 5 service_count = 0 }