Thank you for your response.
For newbies like me: What is a nonce?
This means that if you make a request a request to Boulder and then wait a bit it is possible the nonce you received previously will be invalid when you make the next request.
Please define "a bit". Is that seconds, minutes, hours, or days? When does the nonce (whatever that is) start? When does it end?
Is there background information a newbie like me might read to better understand the failure?
Is the problem that I'm being to fast or too slow when making my requests. I suspect from your answer that I'm too slow but I'm not sure.
I truly appreciate your help but I still have no clue how to solve my problem. Perhaps saying what you (the human) and your code's relationship to Lets Encrypt is might help others and me.
Again. thank you.