Tries to insert NULL into "cuid" about rcmcarddav HOT 2 CLOSED

It sounds like you already found how to enable the debug logging. When debug logging of http communication is enabled, you see the synced vcards inside carddav_http.log. Do all vcards include a UID property? UID property is mandatory for carddav.

If there is a lot of contacts synced and you cannot check all of them, you could also add a check before where storeContact is called in SyncHandlerRoundcube.php that checks if save_data['cuid'] is present and if not creates a log entry with the URI of the card. This you can use to find if there are defective cards sent by the server and which they are. I can provide this code for you if needed.
Edit: Even easier: With debug log level you will find all DB insert statements in carddav.log, so you should see the insert statement right before the error message including the uri and other attributes of the card.

Version 3 vcards returned by the server should not be related to this issue.

Which version of rcmcarddav do you use?

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