We had this error yesterday. Our suspicion is that elasticsearch was out of CPU power without being aware of it. It runs on containers on a powerful machine (32 cores), the container was restricted to 2, but we hadn't yet set the processors - we planned to, and now we'll actually do it. The customer did a load test, and elasticsearch came back with a timeout to most requests. A restart fixed the immediate issue.
We will set the processors parameter this and ask the customer to rerun their test to confirm if this was the issue. Until then it's only our best guess.