Calendar app problems after update to oc 10.0.0

Please help us by providing the following info. Before posting please also check the pinned "Known issues" threads and make sure that you're running the latest available version for your oC release:

Steps to reproduce
1. upgrade per sudo apt-get upgrade/update from oc 9.1.4 to 10.0.0 fails by not disabled app.
2. manual disableing of apps with occ - upgrade works
3. update of calendar app and contacts via market

Expected behaviour
calendar and contacts should work as before
contacts okay, calendar not
Actual behaviour
contacts working fine, calendar app is not accessible via browser (firefox)
IOS sync - no
Thunderbird/lightning is working as before; create and delete events
Android CalDav sync adapter is working as before; create and delete events
Server configuration
Operating system: ubuntu 16.04 LTS
Web server: apache 2.4.18
Database: mysql
PHP version: PHP7
ownCloud version (see ownCloud admin page):
Updated from an older ownCloud or fresh install: 9.1.4
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption):
external storage local

Duplicate. Please avoid cross-posting in different languages.