There may not be an obvious functional problem, however these errors below are seen in the logs. In some cases there will be missing Exchange events.
"Message: Failed to search for mailNickname"
"Message: Can't find owner SID information, mailbox folder "
"Message: Can't find systemMailbox "
There has been one cause of this fixed in the IISHook (changeset 128640) where the domain user name differ from the email address. There's a second cause where the alias is also different than the email address. OWA collects the mail owner as the mailbox name trimmed before the @ sign which the previous contained. When this goes through the pipe the alias is returned which passes the SID look up allowing the event to be audited, but the mailbox name also comes through which fails the search and causes the warning and error above to be written to the agent logs.
Upgrade to ChangeAuditor 4.9.39 or later.