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?

Wow.. no idea on that one. I think Calendar.Setup is related to the CalendarDateField javascript plugin. But that's not related to the EventCalendar module.

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.

Do you have DOM installed?

Yeh, perhaps uninstall that and reinstall it?

You've got the 2.4 (i.e. latest) version of all these modules? Or are you running 2.3?

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

Ok this is getting ridicules. I am not sure what brought it back, but its doing it again.

And yes, running ss 2.4 with all recent modules.
John