Hi, I've been using CalDAV on my Synology server to synch between my desktop Thunderbird instance and handheld without problems. I recently added my laptop as a third platform which synched fine. However now I am working offline (I have an Internet connection but no remote access to Synology), Thunderbird repeatedly comes up with the message: An error occurred when writing to the calendar My Calendar! Please see below for more information. Error code: MODIFICATION_FAILED. The error message is maximally annoying as it steals focus from all Thunderbird windows then hides in the background, making it look like the app has disappeared entirely. I have Offline Support checked in the calendar properties. Platform: Windows 11. I've tried deleting calender_date\cache.sqlte as suggested elsewhere but that makes all my appointment disappear.