Hello forum users - I'm new here and hope someone can help me with this one. I have CalDAV calendars and "tasks" imported into Thunderbird from my main email provider. Both calendars and tasks work fine, both ways, with one exception. Within Thunderbird, I am unable to reassign an event created in Calendar 1, to Calendar 2. I'm also unable to reassign a task created in Task List 1 to Task List 2. The only cumbersome workaround I found was making Thunderbird go offline (bottom left button), then reassigning the calendar or task while offline, then going back online, and then syncing. The error message I get when trying to change a calendar or task category while ONLINE is as follows: ''Error code: MODIFICATION_FAILED Status Code: 2,147,500,037, null Server Replied with 403 If you're seeing this message after snoozing or dismissing a reminder and this is for a calendar you do not want to add or edit events for, you can mark this calendar as read-only to avoid such experience in future. To do so, get to the calendar properties by right-clicking on this calendar in the list in the calendar or task view.'' Changing online/offline support options for individual calendars and tasks did not allow me to change their category while online. Also, none of my calendars and tasks are read-only, nor should they be. Does anyone know why this is happening and how to fix it so that I can change a calendar or task calendar while online?