I've got a OS X Server 10.4.3 (upgraded some time ago from 1.3.9). It has few users (2-3 that routinely use email) and runs web services. Every once in a while lately (2-3 times some days) it will fail to authenticate. As the logs below show it will log users in just fine, and then with nothing in the log stop working. 45 seconds to 90 seconds later all is well again.
I've looked at other logs and don't see any indication of a problem occuring or what is happening to cause it to start working properly again.
Dec 15 10:50:01 server imap: login: host.domain.com user2 LOGIN User logged in Dec 15 10:52:55 server DirectoryService: could not load the replica file. Dec 15 10:52:55 server postfix/smtpd: warning: AOD: Authentication failed for user user1. (Open Directroy error: -14090) Dec 15 10:52:55 server postfix/smtpd: warning: host.domain.com: SASL PLAIN authentication failed Dec 15 10:53:00 server DirectoryService: could not load the replica file.