DAPLink bootloader update / flash problem

I have a problem with both an FRDM-K64F and the K66F board. Both do not work when connecting via USB. Also the bootloader mode is no longer available. Also the attempt to flash the DAPLink bootloader via pyOCD (based on the manual “DAPLink bootloader updates with external debug probe” ) was unsuccessful. Python reported the error “pyocd.core.exceptions.FlashFailure: init error: 536870913”.
Can anyone here give me a hint or help me in any other way?
Thank you.