The lack of detail here makes it difficult to speculate but one possibility is that the active task at the time the Remove method is called is not a member of the root Tasks collection, i.e., is a descendant of some root level task. Tasks collections are nested, that is, the Tasks collection exposed by the UltraCalendarInfo object contains only the root-level tasks, and each of those tasks might have a non-empty Tasks collection.

Note that the Task class exposes a Parent property which represents its immediate parent; in the case where the task is a root-level task, this will be null, implying that the task belongs to the UltraCalendarInfo.Tasks collection. If the Parent property returns a non-null value, you can get the collection to which the task belongs via the Task.Parent.Tasks property.