Can upload program code only one time, then I have to burn bootloader again about mightycore

[MCUdude]

The reason why you can upload the sketch just once is because the your auto reset isn't working. After you burnt the bootloader, the microcontroller is resetting itself every second or so, so uploading a sketch will always work the first time. You should probably check your reset circuity. Use the setup on the first page as a reference

Hi and thanks for the fast response. I was not aware about the bootmanager's self resetting. So, I suspected a software issue. With your explanation it seems now to me, that there is a hardware issue. I will check and find out and report back.

[MCUdude]

Did you figure this out?

I found two faults. The first was a a bad soldering and the second wrong parameters for the fuses.
So, the issue can be closed. It had really nothing to do with your code.