Maixduino no connection , upload unable

I have got a new Maixduino board several weeks ago and at the beginning it worked.
But a few days ago I tested the GPIO´s with Micropython, and suddenly I´m unable to upload any programm. I tested aswell the Arduino IDE and get the following information:
Greeting fail, check serial port ([ERROR] No vaild Kendryte K210 found in Auto Detect, Check Your Connection or Specify One by-p COM3 )

When I connect the USB cable the red LED from the serial input T-K210 is switched on.

What can I do to enter the ISP mode. I tried already the boot and Reset-Pin.

I hope to get some help. Otherwise I have to throw the board away.