This is so annoying. You would expect a pool like nanopool to have a better stratum implementation. The reason you include the "id" in each message is to support an asynchronous flow where the miner can send multiple messages quickly with different ids, then map each response received to the corresponding message sent using the id in each response.

But, Nanopool doesn't support multiple outstanding messages, they always send the id of the last received message. So, I'm guessing what happened was that you sent two shares in quick succession, id 3 and 4. Nanopool sends two replies, both with id 4, which is a broken implementation. We can't map the second reply, because we've already seen a reply for id 4, and fail to process the message.

We will fix this in the next 1-2 days. Supportxmr works fine.

Just so I understand, is the miner still mining ok with this error message, or it doesn't work at all? As long as its for shares only, we will lose track of a few shares, and our poolside hashrate in the miner will be a little lower, but everything is otherwise working ok.