The apps contacts and calendar not working after the upgrade? I had the same problem. Found some help here: https://github.com/owncloud/core/issues/14033

To get my contacts app working again I had to delete it from the server via ftp and upload the upgraded app from here: https://apps.owncloud.com/content/show.php/Contacts+8.0?content=168708

That could work for the calendar app, too, but I didin’t test it. Download it here: https://apps.owncloud.com/content/show.php/Calendar+8.0?content=168707