I had calendar setup and I had an extended mod for the calendar installed.
I removed both folders from root and ran a /dev/build
I hand removed the tables from the DB that were left.
I uploaded the newest version of Event Calendar
Added the Calendar to the site tree and "save & publish" then clicked to add a new calendar event.
And this pops up in a JS alert window.

Calendar.Setup: Flat specified but can't find parent.

Any idea what is going on and how can I fix it?

Yeh, its in the calendar-setup.js file.
I am at a complete loss here. It only happens on the calendar event page. But I have not the slightest clue what changed to make it start doing this. I have re-installed the module 3 times now but I fear I will have to reinstall the site.

I can say that the calendar event cms page doesn't load correctly since the forms are not in the tabs like they should be. Instead all the tabs are lid out on top of each other.

Yeh, perhaps uninstall that and reinstall it?

Whew, reinstalling DOM and Event Calendar worked and fixed it.