Just for closure.. I found the problem. By probing around with a DDM I found an open circuit between the SWCLK pin on the SWD connector and the SWCLK pin on the header pins (& therefore likely also the MCU). I unsoldered the SWD connector and the open circuit remained, so not a cold solder joint. The unpopulated SWD pads on the other side of the board however probed fine. I resoldered the SWD connector to the other side of the PCB and it now works. So, possibly some QC issue during fab?
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