Hi Alice,
thanks for your quick answer XD
No, I didn't mean that I replaced the existing chip, but the situation is pretty much the same as I'm trying to write the content of a brand new device.
I copied the pinout of the existing SWD connector and pasted it to a new board. I used the FRDM to develop a prototype of the firmware to be loaded into the new chip, needless to say that the code is exactly the same, otherwise I would have a different kind of error (device not supported or something similar).
The "old" chip is still programming and working fine, but the "new" one has this problem.
The FRDM programming chip has the latest MBED bootloader as I thought this might have solved this.
I will try to change the bootloader FW to the Jlink as soon as possible and see if this fixes the issue.
Thanks again for your answer.
Have a nice day,
Tommaso