I don’t know why there’s sometimes an exception for the first query, but it seems you can safely ignore it.
About the POST: maybe it’s something with the tool you use to send requests, please see LanguageTool HTTP API to get the curl requests which should work.
thanks for your help, I wrote a simple Java Client to call the service, there everything works fine. When I call it using Chrome every request results in an exception but the result is displayed properly.