I guess that your service adds messages to the queue by query. How exactly it's done doesn't matter too much; the fact is that there always is a piece of code adding messages to the queue (which can be debugged and adjusted).
But now when you mentioned a specific error, that's what we should focus on. Notice that it mentioned your endpoint ID (SendPriceList), therefore it successfully found it. And you see that the batch starts processing the message - it fails during this processing. It disproves the previous theory that messages in the queue are ignored.
