next progress... I removed the whitespaces between the data:
send_msg(0x41, 0x56, 0, 0,'\x20\x00\x00\x00\x00\x00')
ret7 = receive_msg(0xC1, 0x58, 0, 0, 64)
send_msg(0x41, 0x56, 0, 0,'\x20\x00\x20\x00\x00\x00')
ret7 = receive_msg(0xC1, 0x58, 0, 0, 64)
send_msg(0x41, 0x56, 0, 0,'\x20\x00\x40\x00\x00\x00')
ret7 = receive_msg(0xC1, 0x58, 0, 0, 64)
...
Now I get no more errors, but without loop, I manually filled the 80 sends... seems to be indeed a formatting problem in the loop...