That message is generic and always includes all reasons why. Those with 0's don't apply (like the Null one). Those with other numbers apply so which condition applied for you?
Or you can run this query in code and suppress warnings and it should append what records it can and bypass what it can't. You should code it so that if it errors you turn warnings back on with an error handler.
The datasheet view of an action query will at least show the number of records to be affected (depending on what you're doing) so you could scroll down to see the record count if that will give you some clue. In your case that might be thousands of records though. Pretty sure your issue has nothing to do with the record count. The missing clue is which of the 4 (or is it 5?) parts of the message applies here. Could even be more than one reason.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.