I got the same error code (UT000128) because the http request is invalid as it has set the Content-Length but the client does not send the request body correctly. Undertow server would be blocked at a certain I/O thread, and it will get this error code after client close tcp connection.

If you review undertow source code, you will find the details defined by io.undertow.UndertowMessages . This can prove my investigation.

@Message(id = 128, value = "Remote peer closed connection before all data could be read")
IOException couldNotReadContentLengthData();