A user reported the following, so I'm passing it on in case it helps others:
I found the solution to a problem I reported a few days ago. It concerns OmniFocus synchronization using WebDAV in IIS 6.0. You may want to publish this solution on your website or pass it on to other customers having similar problems.
IIS 6.0 returns a 404 error when OmniFocus tries to access its xyz.client file. I don't know why that is, but I found a way to work around this problem:
1. Go into the IIS managament tool (inetmgr)
2. Open the properties of the concerning website
3. Open the "HTTP Header" tab
4. Click on the button at the bottom called "MIME Types".
5. Add a new MIME type using: Extension: .client / Type: Text (see screenshot below)