I try to create a task with an alarm on an CalDAV Network Tasklist within Thunderbird Lightning. The alarm set is ignored and in the thunderbird error console I get following error: Zeitstempel: 02.03.2016 17:36:18 Fehler: Assert failed: aElement 2: [chrome://calendar/content/calendar-ui-utils.js:27] setElementValue 3: [chrome://calendar/content/calendar-ui-utils.js:81] setBooleanAttribute 4: [chrome://calendar/content/calendar-event-dialog.js:554] loadDialog 5: [chrome://calendar/content/calendar-event-dialog.js:297] onLoad 6: [chrome://calendar/content/calendar-event-dialog.xul:1] onload 7: [null:0] null Quelldatei: resource://calendar/modules/calUtils.jsm -> file:///afs/uni-hohenheim.de/hhome/f/faeustpp/.thunderbird/8baql9xg.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/calendar-js/calUtils.js Zeile: 1001 ---------------------- What is the best way to trace the error? I think the fault is about lightning sending an broken caldav "format", because same scenario on an apple client is working. ---------- My Thunderbird Version is 38.5.1 and Lightning 4.0.5.2.