Problem flashing MCB900 board

Regarding MCB900 development board

Problem : I could not flash even a hello world program on the MCB900 any more.

I was able to flash only once.
I followed all steps in the MCB900 manual.

I suspect the crystal booted to the MCB900 not functioning. But I can still run already once flashed program.

I want to know what is the impact on the MCB900 board when soldered with a external crystal.

Thanks

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