Ubuntu 16.04 Calendar is read only via CalDAV

I’m using Evolution on my workstation. I installed Nextcloud 11.0.3 on an other pc with Ubuntu 16.04 and the apps for calendar and contacts. The contacts app works fine, but I got a problem with the calendar app.

I can update the calendar, when I log into Nextcloud by Firefox. All changes are displayed correctly in Evolution after sycronisation.

When I want do any changes via CalDAV by Evolution I get the message that the calendar is read only. (German: Das Ereignis kann nicht bearbeitet werden, weil der gewählte Kalender nur gelesen werden darf.)

I couldn’t find, where to change anything. Could this be a bug?

Over night both computers were switched off. Today I started both, to have a look to the calendars.

There was no read only lock by evolution!

The reboot of both computers fixed the problem.