This year, I had to change some of the headers,
This smells like you changed a data type in the target table and now you can't append from a source where the corresponding field is of an incompatible type. It's a very generic message that provides clues for which you have to really focus on, which is why I'm not suggesting something like key violations (that one is zero). Maybe post a bit more about what's coming from where and what it's going into and how, unless you check out my guess and find that's the problem.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.