calender shares are not possible anymore

[FKBOX]

Owncloud 10.1.0, calender 1.6.2
calender ist working, but I can't share calenders to user or goup. The shares I had before have gone.
owncloud.log:
{"reqId":"OAy3F","level":3,"time":"2019-02-24T09:57:19+00:00","remoteAddr":"88.94.90.54","user":"Frank_K","app":"OCP\Share","method":"GET","url":"/owncloud/ocs/v1.php/apps/files_sharing/api/v1/sharees?format=json&search=ver&perPage=200&itemType=principals","message":"Sharing backend for principals not found"}
{"reqId":"Hqa1BW","level":4,"time":"2019-02-24T09:57:22+00:00","remoteAddr":"78.94.90.42","user":"Frank_K","app":"webdav","method":"POST","url":"/owncloud/remote.php/dav/calendars/Frank_K termine_a1/","message":"Exception: An exception occurred while executing 'INSERT INTO oc_dav_shares (principaluri, type, access, resourceid) VALUES(?, ?, ?, ?)' with params ["principals\/groups\/Grp_B1", "calendar", 3, "3"]:\n\nSQLSTATE[HY000]: General error: 1364 Field 'publicuri' doesn't have a default value:

how can I repair that?

[FKBOX]

General error: 1364 Field 'publicuri' doesn't have a default value! Can anybody tell me what 'default value' I can put in field 'publicuri'?