I'm having this same issue on Ubuntu 22.04 and Fedora 39. Similar to archived post [https://support.mozilla.org/en-US/questions/1354662 Thunderbord Calendar event notification failure to dismiss] Calendar notification box doesn't respond to any clicks to close, dismiss, or snooze. This notification comes up about 5 seconds after opening Thunderbird. Clicking the modal opens crash dialog "Application "thunderbird" is not responding You tried to close window "1 Reminder" from application "thunderbird" (Process ID: 72811) but the application is not responding. Do you want to terminate this application? Warning: Terminating the application will close all of its child windows. Any unsaved data will be lost." The archived post above suggests "disable or remove the add-on, then add the calendar from File/New/Calendar...On the Network..." 1. I used this method when adding my calendar. No add-ins are involved. 2. I only have about 5 seconds between opening Thunderbird and the notification locking it up. Very limited options for any workarounds. 3. Notification has already been dismissed within Nextcloud Calendar but still comes up in Thunderbird. 4. This is a Nextcloud based caldav account.