Hi Philipp Adolph,
Do you secure the chip with your last project? Check the FOPT SEC area on your project, do you secure the chip.
About the problem board, could you change your on board debugger to JLINK, then use the JLINK commander to connect your board, whether you can find the ARM core or not?
About how to change the JLINK firmware, please check this link:
www.nxp.com/opensda

Please try to it on your side.
If you still have problem about it, please kindly let me know.
Have a great day,
Kerry
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