My problem was the sending of the characters I will leave the scan for the Hit any key as it is, because it works at the moment. If it fails I have a lot of alternatives.
I got rid of the VISA OPEN, the buffer flush, the buffer size settings and the 200ms wait-ms. Still the same: After 14 or 13 chars my transmitted message gets cut off. I leave it like (with the delays) it is or try it with another UART and see, if the behaviour depends on the interface.