For german users of OmniFocus:
GMX Free email account with webdav works. At GMX it's called "MediaCenter".
Contrary to open the GMX webdav-server in the finder, as explainded in the
GMX-helpfiles, for syncing OmniFocus with the webdav-server don't use
but instead use
See screenshot...
Upon pushing the "Sync jetzt" button, the OmniFocus data will be synced with the GMX webdav-server, ready to be synced with other Macs, devices, etc...
Jo