I recently thought about switching from my local sync via Bonjour over to webDAV, to be able to sync with my OmniFocus on my home-desk even, if there are problems with local network. I did a quick search and found out that my e-Mail-Provider GMX was offering a webdav-service. So here ist how to set up you Omni-Focus both on iPhone and Desktop to do the switch.
1. Sync Omni-Focus for iPhone
2. Sync Omni-Focus on your desktop
3. Open up your Finder and hit CMD + K and connect to your GMX-Server at the adress: https://mediacenter.gmx.net and hit connect. My machine was asking for some permissions to use information from my keychain to get my login-details for my GMX-Account.
4. You can now create a new folder in which you can store your Omni-Focus-Sync-Database. In my case I created a foder named ofodb
5. Go to your omni-Focus preferences and switch to the Sync-Tab and choose volume.
6. Choose your syncfolder and accept it.
7. Hit sync. Omni-Focus should upload your library after that.
8. Switch to your iPhone and open up preferences there.
9. Go to sync and type in the mediacenter-adresss given above. Make sure you also specify the correct folder there.
10. Omni-Focus will prompt for your login-Details of your GMX-Account – Type them in. (If not, try to leave the folder and just put in this adress: https://mediacenter.gmx.net
11. Sync with your GMX-webDAVserver!

I’ve not tested this feature very much, but there might be some problems with the subfolder ofodb. I’ll try to put my Omnifocus-Database in the root-directory.

Edit
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After some testing it looks like this feature is a bit unreliable when it comes to different network connections. I was unable to sync my iPhone while I was connected via WiFi. Syncing went back to normal, when I disabled WiFi, I could sync via an Edge-Connection.