We are seeing similar error reports from our users - but on shared calendars. Deleting events in one of the connected Thunderbirds does not get synced to the other subscribers in their Thunderbird.

Also don't know if it is related but we see a lot of 503s (reason for DAV error unknown) in the debug log of Thunderbird when creating events too - Thunderbird then marks the calendar with a little warning sign...

Same here. Deleting a calendar event via active sync does not apply the deletion to the CalDav Thunderbird calendar. This is with a regular calendar.

Reporting of deleted events is broken if and only if the User-Agent header matches the substring Thunderbird:

REPORT /SOGo/dav/user/Calendar/personal/ HTTP/1.1
<?xml version="1.0" encoding="UTF-8"?>
<sync-collection xmlns="DAV:"><sync-token>1699861142</sync-token><sync-level>1</sync-level><prop><getcontenttype/><getetag/></prop></sync-collection>

HTTP/1.1 207 Multi-Status
<?xml version="1.0" encoding="utf-8"?>
<D:multistatus xmlns:D="DAV:"><D:sync-token>1699861142</D:sync-token></D:multistatus>

With every other user agent, the expected behavior returns:

REPORT /SOGo/dav/user/Calendar/personal/ HTTP/1.1
<?xml version="1.0" encoding="UTF-8"?>
<sync-collection xmlns="DAV:"><sync-token>1699861142</sync-token><sync-level>1</sync-level><prop><getcontenttype/><getetag/></prop></sync-collection>

HTTP/1.1 207 Multi-Status
<?xml version="1.0" encoding="utf-8"?>
<D:multistatus xmlns:D="DAV:"><D:response><D:href>/SOGo/dav/user/Calendar/personal/3BB0DC-6551D280-1E3-411B3C00.ics</D:href><D:status>HTTP/1.1 404 Not Found</D:status></D:response><D:sync-token>1699861343</D:sync-token></D:multistatus>

This regression is probably caused by https://github.com/Alinto/sogo/commit/f0719ee409dee693e7b4792bd127ccdd4b3043f1.

I verified that it is indeed a problem caused only when the useragent is Thunderbird.

Setting general.useragent.override to empty/null fixes the issue (or better works around it)

Fixed in https://github.com/Alinto/sogo/commit/6b08f2e4decc87ee57609355dee5a6657bca6110

Sebastien