Since owncloud-9.1.2-1.fc25 and owncloud-9.1.3-1.fc25 shared calendars are not editable via CalDAV. I'm using Thunderbird 45.5.1 with Lightning 4.7.5.1 on Fedora and DAVdroid 1.3.4.1 on Android. Editing the calendars via WebGUI is working. This has already been reported upstream: https://github.com/owncloud/core/issues/26667
Solved. There are new sharing URLs. Check the WebGUI for the URLs. More information upstream: https://github.com/owncloud/core/issues/26667