Warum auch immer, hat das Update von Kopano 4.2-5 zu 4.3-2 das php-curl Paket deinstalliert. Genaue Fehlermeldung war:
[Mon Nov 12 18:29:08.878397 2018] [:error] [pid 24048] [client client-ip:33460] PHP Fatal error: Uncaught Error: Call to undefined function curl_init() in /usr/share/kopano-webapp/plugins/spreedwebrtc/php/class.turnauth.php:42\nStack trace:\n#0 /usr/share/kopano-webapp/plugins/spreedwebrtc/php/module.spreedwebrtc.php(100): TurnAuthConnector->getTurnAuthData()\n#1 /usr/share/kopano-webapp/server/includes/core/class.jsonrequest.php(57): PluginSpreedwebrtcModule->execute()\n#2 /usr/share/kopano-webapp/kopano.php(139): JSONRequest->execute(‚{„zarafa“:{„plu…‘)\n#3 {main}\n thrown in /usr/share/kopano-webapp/plugins/spreedwebrtc/php/class.turnauth.php on line 42, referer: https://mail.kundendomain.de/webapp/
Lösung war apt-get install php-curl