I have seen this kind of error on a recent snapshot (don’t know which version) when I added a changed a Switch in a Contact (mqtt) item. I traced it to a rest api call when the UI (In this case Basic UI) was open and requested a sitemap page with the Contact item with a map. When the page refreshed this error was triggered. It might be in other cases this also happens. I was trying to add some debug code, but then I restarted openHAB and the error was gone.
In general I suspect the error is not correctly logged in the rest api calls. The shown error comes directly from jetty. At that point in the code just logs the context information available and rethrows the error. but it’s not logged by openHAB/SmartHome. That is unfortunate because it could show more context information and point cause of the problem. It think it’s very hard to identify what is causing the problem with the current available information. So I guess it first requires some improvement in the logging.