Fix Thunderbird Calendar (Lightning) Not Syncing with CalDAV or Nextcloud

A Thunderbird calendar can keep showing old events even while Nextcloud Calendar has the latest changes. Sometimes Thunderbird is simply offline or displaying a local calendar; other times the CalDAV address, account credentials, or server response has changed. Start by checking whether the same event appears in Nextcloud’s web calendar. That one comparison tells you whether to troubleshoot Thunderbird or the server connection first.

“Lightning” is the historical name many users still use for Thunderbird’s calendar. Current Thunderbird includes calendar features, and Nextcloud documents native CalDAV support in Thunderbird 102 and later. You generally do not need a separate Lightning add-on for a modern Thunderbird installation.

Are events missing everywhere, or only in Thunderbird?

Open the Nextcloud Calendar web app in a browser and look for an event that is missing or stale in Thunderbird. If it is also missing on the server, Thunderbird cannot download it; check whether it was saved to the intended Nextcloud calendar and whether another client can see it. If the event appears in Nextcloud but not Thunderbird, continue with the local connection checks below.

Also check the direction of the problem. If a new event created in Thunderbird does not appear in Nextcloud, confirm that the event was created in the network calendar, not a calendar labeled “On My Computer.” A local calendar is stored in the Thunderbird profile and is not synchronized through CalDAV.

Is the calendar enabled and Thunderbird online?

In Thunderbird’s Calendar view, check the list of calendars on the left. Make sure the Nextcloud calendar is checked and that Thunderbird is not in Work Offline mode. Then restart Thunderbird and allow it to reconnect. A network calendar can retain cached events for offline viewing, so seeing old appointments on screen does not prove that the server is reachable or that a recent change has synced.

Try a small, easy-to-identify test: create a temporary event a few minutes in the future in the Nextcloud calendar, save it, and check for it in the other interface. If you test from Thunderbird, confirm the calendar name in the event editor before saving. Delete the temporary event after confirming whether it appeared on the server.

Does the CalDAV address point to the right Nextcloud calendar?

A typo, an old server name, or a calendar-specific URL copied from a different account can prevent discovery or subscriptions. In Nextcloud, open Calendar and use the menu beside the calendar you want to sync to view or copy its CalDAV link. The URL commonly has a path under /remote.php/dav/calendars/, but the account name and calendar ID vary. Do not guess the complete path from an example.

For a new connection, Nextcloud’s documented method is to create a network calendar in Thunderbird, enter your Nextcloud user name and server URL, select Find Calendars, and subscribe to the calendar you need. If you already have a calendar configured, compare its saved URL with the current link shown by your Nextcloud server.

To add or review a network calendar, open Thunderbird’s Calendar view, choose New Calendar, select On the Network, then enter the account details and server location. Depending on the Thunderbird version and the server URL format, you may either use the Nextcloud server URL and discover available calendars, or paste the specific CalDAV calendar URL. Follow the fields shown by your installed version. Nextcloud documents both the newer discovery flow and a manual calendar URL method.

Do the credentials still work?

An authentication prompt that repeats, or an HTTP 401 or 403 response in an error message, usually means the server did not accept the credentials or account access. Confirm the Nextcloud username, then sign in to Nextcloud in a browser with the same account. If you recently changed your password, Thunderbird may still have the old one saved. Remove the outdated saved credential in Thunderbird’s password settings, reconnect, and enter the current password when prompted.

If your Nextcloud account requires two-factor authentication, check your server’s supported client-login method. Some deployments require an app password for external clients. Create one only through your own Nextcloud account’s security settings or your administrator’s documented process; never paste an app password into a URL or share it in a support request.

Is Thunderbird reporting a certificate or connection problem?

If the error mentions a certificate, secure connection, timeout, or name resolution, verify that the server is reachable from the same computer. Open the Nextcloud site in a browser. If it fails there too, troubleshoot the network, DNS, VPN, proxy, or server availability before changing calendar settings. If the site loads but CalDAV fails, check that the server address is the same host and base path shown by your administrator or Nextcloud installation.

For a certificate warning, check the computer’s date and time and confirm the site’s certificate is valid for the hostname in the calendar URL. A self-signed, expired, or mismatched certificate should be fixed on the Nextcloud server or trusted by the organization using its normal certificate process. Do not permanently accept an unknown certificate to make calendar synchronization work.

Could the event be in a different calendar or hidden by a view filter?

When only some events appear to be missing, check whether the correct calendar is selected, whether the date range is right, and whether the event is assigned to another calendar. Search for the event by title in Nextcloud and Thunderbird. Shared calendars may also have different permissions: being able to see a calendar does not always mean you can edit it. If other users’ changes are missing but your own changes sync, ask the calendar owner to confirm that your account still has access.

Time zone differences can make an event appear on a different day or at a different hour without indicating a sync failure. Compare the event’s time zone and start time in Nextcloud and Thunderbird before recreating it. Avoid creating duplicates until you know whether the original event exists on the server.

Should you remove and add the calendar again?

Re-adding a network calendar can help when its URL or saved authentication is stale, but make it a later step. First confirm that important events are present in Nextcloud’s web calendar. If you have unsynchronized changes in Thunderbird, export or otherwise preserve them before removing any calendar entry. Removing a subscription and deleting a calendar are different actions; read the prompt carefully and do not choose an option that deletes server data.

After preserving local-only changes, remove only the affected network calendar from Thunderbird, restart the application, and add it again using the current Nextcloud discovery or calendar URL. Subscribe to the intended calendar and verify that its name, color, and event list match. Avoid adding the same CalDAV URL repeatedly, because duplicate subscriptions can make events appear twice.

What should you check if you administer Nextcloud?

If the browser works but every CalDAV client fails, inspect the Nextcloud server and reverse-proxy logs around the time of a test sync. Confirm that the Calendar app is enabled, the account can access the calendar, and the proxy forwards the DAV requests and required response headers without rewriting the CalDAV path incorrectly. A web page loading successfully proves that the site is available; it does not by itself prove that DAV requests are routed correctly.

Use the exact URL generated by Nextcloud rather than hand-building a rewrite rule or substituting a sample host. If only one client fails, test another CalDAV client before changing server configuration. That comparison can help distinguish a server-side issue from a Thunderbird profile or credential problem.

How can you verify that synchronization is fixed?

Make one temporary test event in Nextcloud and confirm that it appears in Thunderbird. Then make a second temporary event in Thunderbird, explicitly selecting the network calendar, and confirm it appears in Nextcloud. Edit one test event and check that the changed time or title updates on both sides. Finally, delete the test events and confirm the deletions sync as expected.

A successful check means both applications show the same calendar contents after refreshing, without a repeated password prompt or connection warning. If only one direction works, note which application created the event and the exact error shown by the other. That detail narrows the remaining cause to permissions, calendar selection, authentication, or server-side DAV handling.

Official references

• Nextcloud: Synchronizing with Thunderbird
• Mozilla Support: Creating new calendars in Thunderbird
• Nextcloud Calendar user manual