If I try the same in the Calendar.app it works without any problem. Is this a bug or a user (being me) issue? Anybody else with that problem or a solution?
You can learn about this by choosing “OmniFocus Help” from the Help menu, then navigate to the Preferences page and look for the Notifications section of that page where you’ll find an item with the title “Publish due items as calendar”.
Oh! You shouldn’t have to hunt around for an address…
After you turn on the feature and sync once (so that the calendar gets published—which happens whenever you sync), you should be able to press the Subscribe button.