Your web calendar has current events but Thunderbird looks out of date? The vendor says version 159.0 fixes CalDAV synchronization that could stop after a network error until restart. This guide helps desktop users and small IT teams investigate through read-only checks, without deleting calendars. [1]
What the evidence actually covers
The release notes dated October 27, 2026 explicitly list this CalDAV fix. CardDAV contacts, IMAP messages and Microsoft Graph are separate functions. The separately listed OAuth2 calendar discovery fix is not the same fault. This evidence concerns Thunderbird 159.0 on the Release channel, not every ESR version or provider. A restart helping temporarily does not establish the cause. [1]
Preparation and backup
You need the known affected calendar, approved access to the provider's web calendar and protected backup storage. Record the operating system, running version, installation method and incident time. Find the profile folder through Help → Troubleshooting Information, quit Thunderbird completely and copy the whole folder to a separate location. It contains sensitive account data. An additional ICS export can preserve the locally visible calendar, but may already be stale and replaces neither a profile backup nor a server backup. Do not publish or reimport it for testing. [2] [3]
Five safe checks
- Confirm that the affected calendar really uses CalDAV and is selected and visible. A local calendar or read-only ICS subscription is a different case. Compare two known existing events with the provider's web calendar: date, time, time zone and one recurring occurrence. If they are missing there too, investigate the provider's data first.
- Restore normal connectivity and check that the provider is reachable. Run the usual calendar synchronization and observe whether known events remain missing. Do not deliberately create a network outage. Stop at certificate warnings, account lockouts or permission failures; do not disable checks or repeatedly try passwords.
- Back up first, then use the approved update route. For a direct installation, Help → About Thunderbird can automatically check and download; macOS has the corresponding About Thunderbird menu item. Update Linux packages through their package manager. Do not switch managed or ESR installations to another channel yourself. After the required restart, check the version actually running. [4]
- Open the same CalDAV calendar and repeat the read-only comparison. If synchronization previously resumed only after a normal restart, record that observation, not a proven root cause. Do not preemptively remove calendars, accounts, caches or saved credentials. Do not alter real events or send meeting invitations as tests.
- After another normal application start, check the same events again. Record version, time and result. If a network error later occurs during ordinary use, check synchronization afterwards; a clean start alone does not test recovery from a lost connection. If it still stalls, give support only sanitized errors and the sequence of events.
Acceptance and recovery
The read-only check passes when expected events, times and recurring occurrences match the provider's data. This proves neither write synchronization nor invitation delivery. Use the approved web calendar temporarily if differences remain and stop further changes. Before restoring a profile, preserve the current state separately, quit Thunderbird and follow the documented profile procedure. A local rollback does not undo server changes; do not use a downgrade as a quick repair. [2]
Date and limitations
As of November 1, 2026. Editorial analysis of vendor sources, not a hands-on Thunderbird test. No specific Bugzilla number has been reliably matched, so none is invented. The dated release supplies the news value, not a demonstrated CalDAV search surge. No affiliate links or additional purchases.