arduino uno smd r2: avrdude: stk500_getsync(): not in sync: resp=0x00

Hi
if I try to upload I receive:

Binäre Sketchgröße: 23.448 Bytes (von einem Maximum von 32.256 Bytes)
avrdude: verification error, first mismatch at byte 0x2680
0xff != 0x51
avrdude: verification error; content mismatch

I will be on a business trip all next week, so I have to stop at this time.
Thank you for your support so far.

Wolfgang