"This version fixes an issue that prevents some events from being modified and some reminders from being dismissed in CalDAV calendars with Offline Support enabled – those affected are encouraged to test this build. If you have seen an error message with “this.mItemInfoCache[aNewItem.id] is undefined”, this is you! Be sure to reset your cache, by unchecking the “Offline Support” checkbox, restarting, then checking it again."
This is a lightning pre-1.9.1 issue, see https:/ /blog.mozilla. org/calendar/ category/ lightning/
"This version fixes an issue that prevents some events from being modified and some reminders from being dismissed in CalDAV calendars with Offline Support enabled – those affected are encouraged to test this build. If you have seen an error message with “this.mItemInfo Cache[aNewItem. id] is undefined”, this is you! Be sure to reset your cache, by unchecking the “Offline Support” checkbox, restarting, then checking it again."
Changelog here: https:/ /bugzilla. mozilla. org/buglist. cgi?resolution= FIXED&query_ format= advanced& target_ milestone= 1.9.1&product= Calendar& list_id= 6608373