Thunderbird 24.3.0/Mac OS X Mavericks/Lightning 2.6.4. I have a calendar set up to sync with an Internet cloud service (not Google; not iCloud). This calendar has worked fine for me for months and I have many calendar events and tasks on this calendar. All of those can be modified and I can create new calendar entries, but I cannot create new tasks. On the Task window, in the New Task field, I see an error message that says "Please select a writable calendar". But I can't enter anything in that field. I can create a new local calendar and move existing tasks from the current calendar to the newly-created one, but I cannot find a way to create any NEW tasks on the existing calendar or the newly-selected calendar. Is there someplace I can select this new calendar as the default for tasks? It would be even better to keep tasks on the synced calendar, but I'll settle if need be.