Using imap-2001a-1.72.0 on Red Hat 7.2, and Mulberry v3 on Windows as client. Procmail is configured to filter mail into mailboxes under ~/mail, and Mulberry is configured to check all IMAP boxes in this tree. Occasionally Mulberry reports a problem querying a mailbox name with ".lock" suffix, suggesting that UW-IMAP server is misinterpreting a mbox lock file as a mailbox. (For instance, given a mailbox named "Clarence", Mulberry might complain about accessing a mailbox named "Clarence.lock", which is probably being generated by another imap instance or procmail.)
This is entirely an email client issue IMHO, and not an imap server bug. There are various email clients, including even Netscape Navigator 4.7 which in some misconfigurations wont work properly with the imap server. The solution is to properly configure the email client. If you truely believe this to be an IMAP server bug, please report it directly to the UW imap development team, so they can look into it and give you their official opinion on the matter.