Strange CalDAV behavior with cal on MacOS 10.15.4

I’m facing a real strange behavior of the calendar and acldav respectively when accessing the calendar via CalDAV from MacOS 10.15.4. If a user creates a new event and invites participants everything works smoothly as long as the user (the one who organizes the event) is not in the address book of the invited party. it does not matter which address book on the Mac the organizer of the meeting is in. As soon as there is a local or any carddav address book entry a server error pops up. Checking the nextcloud logs I found out that sabre complains with following error message:

“message”:{“Exception”:“Sabre\DAV\Exception\BadRequest”,“Message”:“Calendar object with uid already exists in this calendar collection.”,“Code”:0,“Trace”:[{“file”:"/var/www/vhosts/digicoach.at/cloud.digicoach.at/3rdparty/sabre/dav/lib/CalDAV/

Anybody else ever had this problem? Any Idea how to fix this?

this is the log file entry:

{“reqId”:“XqhwFSoZR9ow2dCKj@rrmAAAAAA”,“level”:4,“time”:“2020-04-28T18:04:05+00:00”,“remoteAddr”:“81.10.202.127”,“user”:“tg”,“app”:“webdav”,“method”:“PUT”,“url”:"/remote.php/dav/calendars/tg/personal/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics",“message”:{“Exception”:“Sabre\DAV\Exception\BadRequest”,“Message”:“Calendar object with uid already exists in this calendar collection.”,“Code”:0,“Trace”:[{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/dav/lib/CalDAV/Calendar.php",“line”:201,“function”:“createCalendarObject”,“class”:“OCA\DAV\CalDAV\CalDavBackend”,“type”:"->",“args”:[“8”,“d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics”,“BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//Apple Inc.//Mac OS X 10.15.4//EN\r\nCALSCALE:GREGORIAN\r\nBEGIN:VTIMEZONE\r\nTZID:Europe/Vienna\r\nBEGIN:DAYLIGHT\r\nTZOFFSETFROM:+0100\r\nRRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU\r\nDTSTART:19810329T020000\r\nTZNAME:MESZ\r\nTZOFFSETTO:+0200\r\nEND:DAYLIGHT\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0200\r\nRRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU\r\nDTSTART:19961027T030000\r\nTZNAME:MEZ\r\nTZOFFSETTO:+0100\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nTRANSP:TRANSPARENT\r\nDTEND;TZID=Europe/Vienna:20200428T210000\r\nDTSTART;TZID=Europe/Vienna:20200428T203000\r\nORGANIZER;CN=“Tom Cannibal”;SCHEDULE-AGENT=CLIENT:mailto:tom@cannibal.on\r\n e\r\nUID:d8408200-a2c6-4a04-944f-aa78ef5a2bed\r\nDTSTAMP:20200428T175859Z\r\nSEQUENCE:2\r\nX-APPLE-TRAVEL-ADVISORY-BEHAVIOR:AUTOMATIC\r\nSUMMARY:Cannibals\r\nLAST-MODIFIED:20200428T180401Z\r\nCREATED:20200428T175828Z\r\nATTENDEE;CN=“Thomas Gumpinger";CUTYPE=INDIVIDUAL;EMAIL="tg@nottoknow.org”\r\n ;PARTSTAT=DECLINED;ROLE=REQ-PARTICIPANT:mailto:tg@nottoknow.org\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n”]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:1096,“function”:“createFile”,“class”:“Sabre\CalDAV\Calendar”,“type”:"->",“args”:[“d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics”,“BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//Apple Inc.//Mac OS X 10.15.4//EN\r\nCALSCALE:GREGORIAN\r\nBEGIN:VTIMEZONE\r\nTZID:Europe/Vienna\r\nBEGIN:DAYLIGHT\r\nTZOFFSETFROM:+0100\r\nRRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU\r\nDTSTART:19810329T020000\r\nTZNAME:MESZ\r\nTZOFFSETTO:+0200\r\nEND:DAYLIGHT\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0200\r\nRRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU\r\nDTSTART:19961027T030000\r\nTZNAME:MEZ\r\nTZOFFSETTO:+0100\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nTRANSP:TRANSPARENT\r\nDTEND;TZID=Europe/Vienna:20200428T210000\r\nDTSTART;TZID=Europe/Vienna:20200428T203000\r\nORGANIZER;CN=“Tom Cannibal”;SCHEDULE-AGENT=CLIENT:mailto:tom@cannibal.on\r\n e\r\nUID:d8408200-a2c6-4a04-944f-aa78ef5a2bed\r\nDTSTAMP:20200428T175859Z\r\nSEQUENCE:2\r\nX-APPLE-TRAVEL-ADVISORY-BEHAVIOR:AUTOMATIC\r\nSUMMARY:Cannibals\r\nLAST-MODIFIED:20200428T180401Z\r\nCREATED:20200428T175828Z\r\nATTENDEE;CN=“Thomas Gumpinger";CUTYPE=INDIVIDUAL;EMAIL="tg@nottoknow.org”\r\n ;PARTSTAT=DECLINED;ROLE=REQ-PARTICIPANT:mailto:tg@nottoknow.org\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n”]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/dav/lib/DAV/CorePlugin.php",“line”:525,“function”:“createFile”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[“calendars/tg/personal/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics”,“BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//Apple Inc.//Mac OS X 10.15.4//EN\r\nCALSCALE:GREGORIAN\r\nBEGIN:VTIMEZONE\r\nTZID:Europe/Vienna\r\nBEGIN:DAYLIGHT\r\nTZOFFSETFROM:+0100\r\nRRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU\r\nDTSTART:19810329T020000\r\nTZNAME:MESZ\r\nTZOFFSETTO:+0200\r\nEND:DAYLIGHT\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0200\r\nRRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU\r\nDTSTART:19961027T030000\r\nTZNAME:MEZ\r\nTZOFFSETTO:+0100\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nTRANSP:TRANSPARENT\r\nDTEND;TZID=Europe/Vienna:20200428T210000\r\nDTSTART;TZID=Europe/Vienna:20200428T203000\r\nORGANIZER;CN=“Tom Cannibal”;SCHEDULE-AGENT=CLIENT:mailto:tom@cannibal.on\r\n e\r\nUID:d8408200-a2c6-4a04-944f-aa78ef5a2bed\r\nDTSTAMP:20200428T175859Z\r\nSEQUENCE:2\r\nX-APPLE-TRAVEL-ADVISORY-BEHAVIOR:AUTOMATIC\r\nSUMMARY:Cannibals\r\nLAST-MODIFIED:20200428T180401Z\r\nCREATED:20200428T175828Z\r\nATTENDEE;CN=“Thomas Gumpinger";CUTYPE=INDIVIDUAL;EMAIL="tg@nottoknow.org”\r\n ;PARTSTAT=DECLINED;ROLE=REQ-PARTICIPANT:mailto:tg@nottoknow.org\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n”,null]},{“function”:“httpPut”,“class”:“Sabre\DAV\CorePlugin”,“type”:"->",“args”:[{“absoluteUrl”:"https://cloud.nottoknow.org/remote.php/dav/calendars/tg/person

al/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics",“class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/event/lib/EventEmitterTrait.php",“line”:105,“function”:“call_user_func_array”,“args”:[[{“class”:“Sabre\DAV\CorePlugin”},“httpPut”],[{“absoluteUrl”:“https://cloud.nottoknow.org/remote.php/dav/calendars/tg/personal/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:479,“function”:“emit”,“class”:“Sabre\Event\EventEmitter”,“type”:"->",“args”:[“method:PUT”,[{“absoluteUrl”:“https://cloud.nottoknow.org/remote.php/dav/calendars/tg/personal/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:254,“function”:“invokeMethod”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[{“absoluteUrl”:“https://cloud.nottoknow.org/remote.php/dav/calendars/tg/personal/d8408200-a2c6-4a04-944f-aa78ef5a2bed.ics","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/apps/dav/lib/Server.php",“line”:319,“function”:“exec”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/apps/dav/appinfo/v2/remote.php",“line”:35,“function”:“exec”,“class”:“OCA\DAV\Server”,“type”:"->",“args”:[]},{“file”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/remote.php",“line”:165,“args”:["/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/apps/dav/appinfo/v2/remote.php"],“function”:“require_once”}],“File”:"/var/www/vhosts/nottoknow.org/cloud.nottoknow.org/apps/dav/lib/CalDAV/CalDavBackend.php",“Line”:1069,“CustomMessage”:"–"},“userAgent”:“Mac+OS+X/10.15.4 (19E287) CalendarAgent/930.4.1”,“version”:“18.0.4.2”}