Recently I discovered that I have a lot of contacts which are not in sync anymore with the CardDAV server. (Maybe related to 0003736 )

I'm using the current Thunderbird versions on two machines syncing to a Synology NAS with their CardDAV Server 6.0.3-0078. When recognizing that problem I was using SOGo connector 31.0.2 , then updated today to 31.0.3 and now I'm using the current nightly 31.0.4-2253f36fb2 (after reading a bug report here recommending the nightly build).

Some contacts won't sync from Thunderbird to the server even when a field is changed again in Thunderbird. When moving such a contact (in Thunderbird) to a local adressbook, waiting for the old version on the server got also deleted and then moving it back to the synchronized addressbook, I get following error in Thunderbirds error console:

Precondition failed for card: https://synology:443/addressbooks/users/user/addressbook/C675C6AC-DD20-0001-18A5-2F10AFF010C0.vcf.
HTTP Status Code:412
Redownloading!

The error output from Thunderbird at the terminal is:
*** new sync: 6
6/sync with https://synology:443/addressbooks/users/user/addressbook/...
GOT STATUS: 207
card set for upload: C675C6AC-DD20-0001-18A5-2F10AFF010C0.vcf
ctag matches or drop operation
NOTICE: uploading modified vcard with etag: "930d20012a2eefda3dc6bd8c3edf3e3f"
GOT STATUS: 412
xmlRequest: received status 412 - precondition failed for url: https://synology:443/addressbooks/users/user/addressbook/C675C6AC-DD20-0001-18A5-2F10AFF010C0.vcf
sogoWebDAV.js: an exception occured
Buggy situation! (pendingOperations < 0)
undefined:undefined

stack: undefinedurl: https://synology:443/addressbooks/users/user/addressbook/
GOT STATUS: 207
sogoWebDAV.js: an exception occured
Buggy situation! (pendingOperations < 0)
undefined:undefined

stack: undefinedurl: https://synology:443/addressbooks/users/user/addressbook/
GOT STATUS: 404