Hi Florian Lebrun,
I find you already make this post as assumed answered.
Do you have solved the problem or not?
If you can enter the debug mode, the problem should in the clock configuration area, you can check your board the clock configuration, you must make sure your system clock is correct, then configure the uart with the according UART source clock.
If you still have problem about it, please kindly let me know.
Have a great day,
Kerry
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