On Windows, when a process running a QLocalServer is blocked (e.g. waiting for a client to finish) client processes can successfully connect with QLocalSocket::connectToServer, but writing data to the socket fails. Calling QLocalSocket::waitForBytesWritten results in a timeout and the data written to the socket is lost. On unix this works as expected, the server process is able to read the data sent by the client after it unblocks. The attached archive contains a minimal sample.