Yes it was always the same bit often in a regular pattern but indeed it was a failing USB-TTL converter.
I changed the converter (yes i should have done that earlier before bothering you) and now was able to flash the unit without a problem. i did not suspect the usb-converter as it was used for other things and works well there but maybe the timing is more critical as the baudrate is quite high.
in any case thank you for sharing your knowledge and time and i hope i have more clever questions next time !
THANK YOU !