Microsoft Entourage error -18576 as it happens

I have had a problem that has repeated since I first began using Entourage as an Exchange client since at least version 11.0. I’ve never been able to solve it, but let me at least explain it here, because I’ve seen other people encounter it as well. It’s error -18576, and I hate it.

The symptoms are that I have calendar events displayed locally in Entourage that do not appear to synchronize to an Exchange mailbox. The Exchange mailbox, or account, does have some but not all of my calendar events.

There doesn’t seem to be any rhyme or reason to events that are missing. Many of them are recurring entries on my calendar. Beyond that, I’ve had events I put on my calendar myself, events that have been created through invitations from others, events with and without categories assigned, events that were created manually or with AppleScript, events that are flagged private, and so on.

The sequence of events I see in a traffic capture for affected events goes something like this:

1. Entourage tries to LOCK (a WebDAV call?) the event at a URL path ending in /exchange/userid/Calendar/eventname.eml.
2. Exchange returns an “HTTP/1.1 401 Unauthorized, NTLMSSP_CHALLENGE” response.
3. Entourage tries to LOCK the event again, at the same URL path.
4. Exchange returns an “HTTP/1.1 201 Created” response.
5. Entourage attempts an HTTP PUT of the event at the URL.
6. Exchange returns an “HTTP/1.1 200 OK” response.
7. Entourage attempts an HTTP PROPPATCH of the URL.
8. Exchange returns a “HTTP/1.1 207 Multi-Status” response.
9. Entourage attempts to UNLOCK the URL path.
10. Exchange responds with “HTTP/1.1 424 Method Failure.”

That’s it, that’s the extent of the HTTP/WebDAV exchange between Entourage and the Exchange server. The Entourage Error Log shows an -18576 error, but as of Entourage 2008, has no further description. Entourage’s Exchange-related errors are numbered in the -18000s and are typically based on HTTP (see the Entourage Error Page; use the equation 19000 + error number to determine its HTTP equivalent.

In my case, this is an HTTP 424 error, or a “Failed Dependency.” According to RFC 4918 on WebDAV, this means “The property change could not be made because of another property change that failed.”

The end result in Entourage is an event that does not appear in the Exchange calendar, but does appear locally — and there’s that entry in the Entourage Error Log, of course. There’s one such for each attempt to sync an affected event, so they build up over time — but some events do synchronize. It’s more aggravating that I’m left with a feeling of being in limbo, not knowing whether any calendar I’m looking at is correct or not. (Is there a sniglet for that? Update: I hereby propose “calendremma.”)

It doesn’t seem to matter whether Entourage is connecting to a front end or back end Exchange server (at least through Exchange 2003). Rebuilding the Entourage database doesn’t clear up the problem, although seems to occasionally have offered some temporary respite. However, the events that didn’t sync before still don’t sync.

The events which don’t synchronize fail to appear via Exchange ActiveSync on my iPod touch, and aren’t visible in either Outlook Web Access or Outlook for Windows.