Dito after having installed a new instance on a stable Debian server and imported my old calender.
It occurs every time Thunderbird warns me of an upcoming event and I want to dismiss it. Here is the log output from NC:
> Sabre\DAV\Exception\BadRequest: Calendar object with uid already exists in this calendar collection.
/var/www/nextcloud/3rdparty/sabre/dav/lib/CalDAV/Calendar.php - line 201: OCA\DAV\CalDAV\CalDavBackend->createCalendarObject("7", "c63477e3-22 ... s", "BEGIN:VCALE ... n") /var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 1096: Sabre\CalDAV\Calendar->createFile("c63477e3-22 ... s", "BEGIN:VCALE ... n") /var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php - line 525: Sabre\DAV\Server->createFile("calendars/t ... s", "BEGIN:VCALE ... n", null) <<closure>> Sabre\DAV\CorePlugin->httpPut(Sabre\HTTP\R ... "}, Sabre\HTTP\Response {}) /var/www/nextcloud/3rdparty/sabre/event/lib/EventEmitterTrait.php - line 105: call_user_func_array([ Sabre\DAV\ ... "], [ Sabre\HTTP ... }]) /var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 479: Sabre\Event\EventEmitter->emit("method:PUT", [ Sabre\HTTP ... }]) /var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 254: Sabre\DAV\Server->invokeMethod(Sabre\HTTP\R ... "}, Sabre\HTTP\Response {}) /var/www/nextcloud/apps/dav/lib/Server.php - line 301: Sabre\DAV\Server->exec() /var/www/nextcloud/apps/dav/appinfo/v2/remote.php - line 35: OCA\DAV\Server->exec() /var/www/nextcloud/remote.php - line 163: require_once("/var/www/ne ... p")
If there is any more info I’d be happy to oblige 
Otherwise everything is functioning perfectly …